The MFP activity level is meaningless when you enter your own TDEE or TDEE with Deficit goal down the road. As is the weight loss goal. Those 2 will affect MFP's math on loss amounts, but if you enter an eating goal that includes exercise, and MFP never expects exercise to be there already - the math is always going to be off anyway.

MFP activity levels are NOT TDEE either, their are non-exercise maintenance. They would only be TDEE if you actually did NO exercise.
That's why MFP method you exercise, you increased your maintenance for the day, you log it, you eat more. Same deficit in place.

If you only had a very reasonable 1/2 lb weight loss goal, then I gotta tell you, you were likely eating over maintenance, or TDEE.

And I doubt you were stressing your system with a mere 250 calorie deficit.

The only way that would be stressful to your system is if you did NOT eat back your exercise calories, and you exercised a lot, actually creating a really big deficit overall.

If you were eating back exercise calories correctly, then likely you over-estimated that burn and were eating back too much, or your logging isn't as accurate as you think.
It would be easy to wipe out a small 250 cal deficit with a combo of those things.

So your routine was pretty good.
Did you eat back exercise calories during the time you gained weight?

What does it look like you really ate in total over that time on average, not net if you did, but gross?

It all hinges on that exercise eatback.

If you did, I'm going to suggest you've been resetting and eating in surplus for awhile now. No need for it now.
If you did not, you created a much bigger deficit than 250 and reset may indeed be useful.

So you already experienced the fact you could eat 1370, not gain or lose.

Then eat 1690, not gain or lose - thus proving 1370 wasn't true maintenance.

So it's up in the air if 1690 was true maintenance, or just as suppressed as 1370 was.

Is non-exercise activity for 40 hr work week indeed sedentary?

Regardless, you had deficit from exercise and not always eating back some or any calories.
Except no change in weight, so not a real deficit, so the 1690 wasn't true maintenance then.

And for 12 days 250 cal deficit plus some from exercise, at least down to BMR. Which is what your body would like to burn if you just slept all day - except you don't.

One comment, be aware unless you have tested VO2max and HRmax stats to put in to HRM, it's not accurate for calorie burns. Could be, could very much not be - don't know.

Except you've had enough of a deficit in place, you are no longer burning for maintenance, what you used to.

That's call metabolic adaptation. The body has responded to steeper than desired deficit by slowing you down likely, and making you more efficient.
Meaning you no longer burn what you used to, or could be, in daily life.

And any moments of binge, splurge, big meal or day, ect, is all surplus, not a raise to the metabolism. So slowly increasing fat likely, though some of it is probably carbs too if you see big increases that slowly drop off.

So yes, reset would be useful. Perhaps not as long since your deficit wasn't as severe, though it was lengthy.

Last exercise break at Christmas too? What are the workouts and frequency?
Exercise break might be in order too as you increase.

Figure out your TDEE for walking only during normal workouts.
Eat that TDEE while you indeed only walk.
Then go back to normal workout routine and figure TDEE for that, and increase calories another 100 daily for that week.
Then again week after, another 100.
Keep increasing until you hit estimated TDEE for your routine. 2 weeks there and confirm no weight gain.
Valid weigh-in days, morning after rest day eating normal sodium levels, not sore from last workout.

Then spend 2 more weeks as test, eating 250 more daily yet again.
If truly eating at TDEE, you would gain 1 lb slowly over that 2 weeks, confirming prior level. Then take 15% deficit off prior eating level.
If faster or more gain, water weight and it wasn't TDEE.
If no gain, then wasn't TDEE either.
Test with 250 more again.

I think the spreadsheet is best, as it takes into account the time spend doing different levels of exercise. And those levels mainly based on comparing to walking level, so easy compare for most.

When you think about it, it's obvious 1 hr of walking doesn't burn the same as 1 hr of running as 1 hr of lifting. Start throwing different times in there too, then you see how rough "3-5 hrs of exercise a week" really is rough. And though you can do that and then base results after a month to get new TDEE figure, I figure why not start at best estimate if possible.

Would someone walking 1 hr daily really be Very Active or more? No.

Confirm you have correct weight and age prior to HRM self-test. Also selection of athlete profile in stats. And see what maxHR you've ever seen in a workout, and confirm your HRmax stat is at least 5 above that if you recall it being a hard effort worthy of being called max.

Just stay on Simple Setup tab, and Progress tab, and be honest with Activity Calc you have more than 45 hr sedentary desk job you are active at.

Heya, sorry to highjack your post. I dont know if I need to necessarily reset my meta, but just in case I would like to know how much I burn just doing my work at school (teacher), I want to confirm the method.

So to get it straight:
I do my everyday normal stuff (not extra gym stuff).
I eat my BMR for 2 weeks, see if theres a weight gain.
If no weight gain, eat 100 calories more for 2 more weeks and go on and on like that.
Once weight gain in happening, thats the Non gym TDEE of my Normal day (no extra gym stuff).

Then introduce gym stuff:
If I burn 100 cals at the gym at one time, add extra 100 ontop of Non gym TDEE. Try for 2 weeks, no gain, add extra 100 doing same work out for another 2 weeks etc.
Keep going till gain is seen. Once weight goes up, thats the +Gym TDEE. -10/15/20 etc % for deficit.

Wow, long process, but its a must.

Have I cracked it?

Wow - never heard that method anywhere.