same username there.

But haven't investigated how to do much beyond PM there, which can be done on MFP.

And I don't do any step or VAM goals. My workouts are what they are (always VAM), I'm not taking away from that time to get steps in towards a goal not useful to me.

So likely not inspiring for step competition.

You are correct on walking to a point.

It should become easier for you. Showing you've become more fit.

But if the weight is the same, you'll burn the same calories. I doubt something you've done since a little kid is going to all of a sudden become really more efficient. So no change their.

And how hard or easy it is has no bearing on calorie burn. That 15 lb weight on the ground requires the same force and energy to break it free from gravity, old or young, male or female, scrawny or muscular. Those other things just determines how easy it feels.

Same with walking at same weight, same calorie burn.

Now, you start losing weight, indeed less calorie burn. And actually as far as fitness is concerned, it becomes less of a workout for you.
Less mass moved, less calories burned, less need for oxygen, less need for the muscles. If walking was it, and no increase in intensity, you will lose muscle and fitness eventually as weight is lost. Along with the fat of course.
That would be like losing 10 lbs of weight, if squat and deadlift didn't go up by 10 lbs, then haven't you in essence made it easier, going backwards in progress. And in a diet, you could lose the unneeded muscle.

So Fitbit correctly handles that - you weigh less, you burn less.

Now, what doesn't change that could depending on your starting point. Stride length. More mass likely takes smaller steps, less mass could easily have longer stride.
Fitbit default to calculated stride length based on height, which doesn't change as weight is dropped.
So manually corrected stride length is good to have, especially as ability to step farther improves, especially on running stride.