Encouragement - couple lbs of muscle is nothing to wave a stick at - whatever that means. Great work.
Good metabolism improvement for sure.

Did you maintain weight the last 2-3 weeks?

Don't be obsessed with the scale, only pick 1 valid weigh-in day to minimize water weight changes and use only that one.

So to confirm, you do realize whatever estimated TDEE you were eating at that did NOT include cardio, is NOT the TDEE you will take a deficit from?
The new TDEE with cardio included will be higher, so you likely could be eating at the same amount depending on how much cardio you will be doing.
Is it gentle cardio at least?
Because adding something intense when you go in to a diet is the worst possible time, because in diet recovery is already impaired, no need adding back more intense that needs recovery.
Then again, you could mean daily walking, which is fine on both counts.

Are you manually inputting at the Fitbit site for your Strength Training so it has a chance of giving you best estimate of daily burn?
Probably why you use it right, to know your daily burn, just confirm you correct it for stuff it badly underestimates on. Or what's the usefulness then.

Are you using the Fitbit to give you a weekly TDEE figure to base the next week's eating level on? (hence the encouragement to correct it).

Since you comment about creating large deficit - don't want you doing that subconsciously and getting same bad results.

Just remember this time compared to past times, you are going to fuel your workouts for best improvements, in a diet anyway, and keep a reasonable deficit so you can have sustained loss.
And only enough scale weigh-in's to confirm the TDEE is still up there.

Is your % deficit close to 250 or 500 block calories? That would at least allow setting up Fitbit to get close to the same eating goal figure.

I do enter my strength training based on my polar watch. I have done Insanity and I love HIIT training on a treadmill. Is that too much too soon? Right now I set my calories at my BMR and was letting Fitbit adjust my calories, but I was using my previous weeks average to meal plan....but I did not think about the increase from cardio! So right, I will be about the same.

Adding back Insanity and HIIT - 2 intense loads of exercise on muscles also being strength trained?
Where's the rest for recovery and repair, never mind the fact of going in to a deficit and that being slower - where would it be anyway?

You have also synced the 2 incorrectly then, if I understand your comment about how you have had it setup the whole time.

MFP is comparing Fitbit's total burn for the day (say 2500) to MFP's non-exercise maintenance it calculated from BMR x activity factor (say 2000).

The difference (500) is then applied to the eating goal (say BMR of 1600), whether the one it did because of your selection of weight loss goal, or a manually entered one.

So you have been asking MFP to compare apples and apply the difference to oranges.

So with above numbers, eating goal of BMR 1600 gets a 500 cal increase to eat 2100, but Fitbit TDEE was actually 2500.

So you have NOT been resetting if that was your method the last 12 weeks, because the eating goal was NOT set to maintenance, so that MFP could adjust it up or down depending on how much you did.

Now, to that add the fact that the HRM calorie estimate for weight lifting is wrong use of tool, and totally inflated.
HRM calorie burn is ONLY valid for steady-state aerobic where HR is the same for 2-4 min.
Lifting and HIIT (can't really do that on a treadmill BTW) and Insanity ("intervals upside down") are all non-steady-state and anaerobic if done correctly - totally opposite.

500 deficit would be appropriate if your TDEE was 3333 on average daily, which could be possible, but improbable.

So, either you massively lucked out that your exercise was exactly inflated the same amount your eating goal was synced incorrectly too low, or you still had too much deficit and your system never sped up, and you are still eating at suppressed TDEE.

If you indeed had your BMR set as eating goal last 12 weeks, then you need to figure out how badly did that mess things up.

What is your MFP Mifflin BMR?
What did MFP say your calories burned from daily activity was on Home - Goals?
What was your average TDEE from Fitbit over those 12 weeks (despite the fact it was inflated from manually logging lifting and Insanity and HIIT)?
What is that difference with Fitbit TDEE - MFP maintenance?
Add that to BMR and that's about what you've been eating.
What is Fitbit TDEE - what you've been eating?
That's how much deficit you should have had.

And you lost no weight this entire time from that max 15 gained?

DANGER! DANGER! INFORMATION OVERLOAD. LOL.

- I had my calories set at TDEE for my entire reset. I compared MFP and Fitbit to stay at TDEE. I had my Fitbit goals set to maintenance so it properly adjusted MFP everyday. I think?

- I did not lose during reset. I tested bodyfat/weight every two weeks.

- My current plan is Insanity or HIIT on non-STS days. That's 5 days a week working out with rest days twice a week. During reset I strictly did STS and Abs.

Your goal weight loss or maintenance set on Fitbit has NO bearing on the math done on MFP.
Fitbit reports daily burn to MFP.
MFP compares it's non-exercise maintenance based on your selection of activity levels.
The difference is added or subtracted from your eating goal.
If MFP had been set to maintain, and the given eating level used, then that would all work correctly.

But, BUT, if you had your eating goal set to BMR, the difference was not added to MFP maintenance figure, it was added to your manually setup eating goal of BMR.
There is already a difference between BMR and MFP non-exercise maintenance.
You would still be causing a difference.

I guess to keep it simple - were you eating in total what Fitbit said you were burning in total?

And going forward, if you want single daily goal - you'll need to unsync accounts. Just manually correct the workouts on Fitbit site, and keep an eye on what the TDEE is looking like.

So your workout routine will not be optimal.
Are you going to be using the same muscles in intense fashion one day after another?
Would you lift 2 days in a row with same muscles?
Would you do HIIT 2 days in a row with same muscles?

Is the HIIT because you heard it helps burn fat?
Why the Insanity?

Thanks...I have done Insanity for Years. Since it first came out. I just like it. For the better part of 6 months I have eaten 1500 calories and done insanity and P90X doubles 6 days a week to try to get off my plateau. Embrassing the EM2WL lifestyle has been the hardest thing I have ever done. So I am trying my best to not "over do it" and stick to the program.

Based on what you just said I think I will set MFP to my average TDEE less 15% for the last 3 months. Then I will enter my exercises in both and compare the difference to see how many calories to eat back. I'm a nerd so I don't mind doing this daily. Thanks for all your help!!

You are overcomplicating this.

So you are willing to do it MFP style, eating back calories when actually burning more that day. Might as well use MFP and Fitbit sync the way it was designed then.

To follow the eating more than bare minimum, or having a reasonable deficit, is to do just that.
Pick a reasonable deficit.

How do you do that?
Use that last month's Fitbit TDEE (3 months too big, seasons changes, you get more active, you aren't doing that workout now, ect).
How many calories is 15% of that TDEE?

Go in to your MFP Diet/Fitness Profile setup.
Set non-exercise activity level correctly and honestly. Really Sedentary, really Lightly Active? You can adjust that later if you get big positive adjustments even on non-exercise days.
Pick the weekly goal loss that is reasonable by using one that is close to that 15%.
1/2 lb weekly is 250 deficit.
1 lb is 500.
1.5 lb is 750.

Set MFP and Fitbit to sync positive and negative. Don't worry about Fitbit goals setup, those numbers not used on MFP. Unless you do look at those stats, in which case select the same goal loss amount on Fitbit.

Go in to your Goals to customize your macros - leave eating goal alone.

Now log the non-step based exercise on Fitbit or MFP - but NOT both. It will sync back and forth just fine, and change the Fitbit TDEE just fine, and that will allow MFP to change the eating goal just fine.

You will have the same deficit amount daily, but since your TDEE will go up and down likely, the % will change probably anywhere from 10-15% actual deficit.

Now that's reasonable, and easier. And after a few days, you should be able to plan your intake correctly because you'll know just about what it's going to be.