Can someone help me understand a change in my base calorie goal ?

Congratulations on your excellent results so far: That's great!

First, double check your profile settings carefully. Typos are possible for any of us.

Second, MFP recently changed the calorie estimating formulas based on newer research. Most people going back through guided setup find the changes will slightly increase their calorie goal.

Your increase is not "slight", though, and it's for a faster loss rate besides. That made me wonder about typos.

If I were you, I'd use my most recent month of logging and weight change to estimate a personalized new calorie goal, and input it manually, rather than letting MFP estimate. I've got to admit, I also wonder whether it's ideal health-wise to speed up weight loss when there's less available fat still to be lost, but you didn't ask that, and it's entirely your call.

Wishing you continuing excellent progress toward your goals!

I'm not sure we'll ever know why you suddenly got that high goal and now a lower one again - a glitch perhaps. But I can tell you that it's good to regularly go back to your goal settings to confirm them (just change a setting and change it back) since MFP doesn't automatically update your calorie goal as you lose weight (entering weigh-ins isn't enough to trigger an update).

In normal circumstances, that would have given you a calorie goal a few hundred calories lower than the original one. But since they also updated the activity level setting, that basically gave you your original calorie goal again.

You haven't given us any info about yourself except some info on how active you are (unless I missed it) - sex, age, height, current and goal weight would be relevant? But if 1900 is no longer giving you results, and especially since you've been following the same calorie goal through 40lbs of loss, 1670kcal sounds quite sensible.

1670 sounds great if you stalled on the other number. If it’s too aggressive and you’re hungry, just move in that direction more gradually.

Yes, you can set your calorie goal manually. I have to do that, because MFP's estimates are wildly inaccurate for me (not because of anything wrong with MFP, but because I'm a statistical weirdo). Loosely, instead of going through guided setup, you just edit the calorie goal number. Where/how to do that is different in web browser MFP vs. the phone/tablet app, so if you need more details, please say which version you're using and I'll be more specific.

If your logging has been quite tight already - weighing food, not using other people's recipe-type entries ("meat lasagna - one plate" kind of thing) but making your own meals/recipes, logging every day even ones you might rather forget, every bite/lick/taste/beverage/condiment/cooking oil/etc., then you can use your most recent 4-6 weeks' logging data and bodyweight to estimate your maintenance calorie needs, then substract calories to generate your weight loss.

It's not hard to do, but rather than type it all out here, I'm going to point you to a post in the maintenance section that explains it in detail (because I'm lazy and want to save typing). That's here:

Use method 3 in that post, "Estimate from History" to get a personalized estimate of your maintenance calories, then subtract 500 per day for a pound a week estimated loss rate (or apply arithmetic to the 500 for other loss rates, like half of it, 250, for half a pound weekly loss). Stick with that for another 4-6 weeks to reality test it. (If you were female of the relevant age/stage, I'd suggest using one full menstrual cycle of data rather than 4-6 weeks, but you're not.)

That estimating process worked very well for me during loss, and continues to do so during maintenance, even though my calorie needs differ significantly from what MFP and my good brand/model fitness tracker imagine would be true - hundreds of calories. 🤷‍♀️ Almost ten years of successful logging and weight management experience suggests my estimate is close to right, and theirs are very wrong. 😉😆