BMR is sleeping level calorie burn.
I'm assuming since you walked you must have waken up, which means you burned more than BMR, even though that is your biggest burn of the day.
So easily 1700 calories burned being alive. Plus daily movements.

Fitbit saw 1701 burned at basically 5 pm, 17:00 hrs, or 17/24 = 71% of day gone.

So 29% of day left to go. But MFP uses it's burn rate then.

MFP 1450 eating goal + 250 cal deficit perhaps = 1700 MFP expected daily burn. (are you short and pretty light already? Is BMR 1360?)

MFP 1700 x 29% day left = 493 cal for rest of day if you keep at selected MFP activity level.

Fitbit 1701 till now + MFP 493 rest of day = 2194 estimated daily burn.

2194 estimated - 1700 MFP expected = 494 cal adjustment.

Difference is in the minutes and math rounding.

I'm betting you picked sedentary on MFP activity level, unless you walked a good amount - that just means you are not sedentary.

So MFP had to put the result of it correcting itself somewhere.

Since that correction would effect the eating goal as it should, they just put it under Exercise Diary.

But that adjustment is merely the difference between a more accurate device's estimate of calorie burn, and what MFP estimated you'd burn by your selection of activity level.

You could have no exercise but high daily activity and get big adjustment.
You could have a hard workout and be lazy rest the day and have no adjustment.

So MFP isn't saying you eat 1700 because of 20-30 min of walking, but 20-30 min walking AND all the other activity you do during the day that makes you burn more than it initially guessed.

With 15 lbs to lose, and small, you are correctly at the point of only a 250 cal deficit.

Unless Fitbit has your stride length waaay off and you do a whole lot of steps during the day in activity - you likely are burning a good amount and should be eating more.

But your food logging may be inaccurate - it should be by weight since calories is per gram, not cups or spoons.

With having low TDEE and near the goal, you have less room for inaccuracy to still see results.

Or you are stressing from undereating so much and gaining cortisol induced water weight same time as losing fat weight.

Are measurements still changing?
Hope scale isn't the only thing defining your fat loss, because losing muscle mass is other reason for Fitbit overestimating calorie burn.

New routine and moving into summer are exactly the kinds of things to increase LBM with good water weight (stress related isn't good). You need that increased blood volume, interstitial water, increased stored carbs.

Don't suggest lowering calories more.

Actually would suggest eating closer to TDEE with no diet for a break. Teach your body this is new eating level, no need for stress. Let new workouts cause maximum improvements.

Then take that 250 deficit after a month.

Only other thing would be confirming Fitbit is as accurate as possible, for what MFP is correcting itself with.
Like walking a known distance at your average daily pace (not exercise pace) and seeing if it's correct.
Manually logging any workouts a HR-based Fitbit is going to overestimate because it's not steady-state aerobic exercise, if done long and frequently enough to matter.

Well, literally right now - your TDEE is whatever you are eating. Because you are maintaining.

Unless - you are indeed gaining stress related water weight same time as losing fat, but clothes show that up usually.
That amount of water weight can be upwards of 20 lbs though, perhaps not for you though being shorter.

But stress from life, diet, disease, lack of sleep, unrecovered workouts, ect - can add undesired water weight.
And usually a great sign that whatever is causing the stress is bad news for body overall.

Sounds like your body is adapting right along with you eating less and less.
Have you noticed the Fitbit daily burns going down over those same times of eating less?
First thing body does is have you move less to conserve calories when you scare it eating too much less.
Then it'll slow things down like hair/nail/skin growth, then keeping warm, ect.
Finally it'll slow your base metabolism down.

Now, you can keep eating less and less and eventually overcome those adaptions - but guess how stressed out the body is at that point trying to adapt to less calories and you go even lower.
Forget workouts being useful, body's not about to make improvements that require more calories when it's already not getting enough.

Better to get body burning full blown again, hope you didn't lose too much muscle mass. Which means slowly increasing calories back up to potential TDEE the Fitbit is estimating.

For stride length - high school track is best bet - just confirm what lines are for 1/4 mile, since many have metric markings now.
Start a workout as you walk up to the line, do 1/4 or 1/2 mile at normal daily pace - not exercise pace, not grocery store shuffle. it'll feel slow, but that's right.
Stop workout when you cross line again.
Look at the record for steps done, and it's distance for comparison. Was it close?

For 1/4 mile take 1320 / steps = decimal feet (like 2.1)

In Fitbit settings, walking stride length put feet at 2.
put inches at 12 x 0.1 (remainder in this example) = 1.2 decimal inches.

Sync device and it'll have new info.

When did you finally switch to 250 cal deficit?
What was prior deficit and for how long, how much weight lost with it?

I'd suggest that if you are totally sure you aren't eating way less than 250 deficit, workouts are still good energy and as hard as you want them - then the eating aspect may be off.

In which case it may appear you are eating less and less into danger zone, but not really.

The reason more exercise worked is you burned more than you ate. Considering how hard it can be to burn an extra 500 calories compared to just not eating it - calorie control is the main method.

But if more exercise allows you to eat where you are and adhere better, then that works too.

Do you have or can you have good resistance training as part of what you can do?

The benefits of that are just great for weight loss and maintenance.

The only problem is it doesn't burn a lot of calories during the workout, so you don't get to eat as much as some heavy cardio.

But since a good session burns about as much as a strong walk, it'll be much more body transforming, as well as not allowing a diet to cause muscle mass loss.