Mask tracking weirdness?

What am I missing here? I've used mask tracking before, although not often, and I don't remember it behaving this way.

I'm on Pr 22.5, Windows 10.

I'm tediously tracking a mask, one frame at a time, making adjustments on each frame. When I go back to the beginning of the clip to watch, the mask is no longer what I drew. ??? In other words, the keyframes appear to not stay the same. The mask is usually a slightly different shape, and moved over to either the left or right. The points of the mask move to seemingly random nearby points. And the part of the video I worked so laboriously to cover is once again visible.

I have tried more than once. The first time I spent an hour on a short clip, and then assumed I had done something wrong, so I tried again. Another hour ish. Same thing. The points of the mask do not stay where I draw them. I don't remember this happening before. It's either a bug, or operator error somehow. I'm open to either, just want to know what's going on.

To be clear: when I go back, I'm not tracking backwards... I'm just moving the playhead to the beginning of the clip to watch what I accomplished. And that's when I see the mask keyframes have changed.

Please help! Thanks in advance.