Rotobrush expansion after couple frames

Hello, i'm having this same issue: https://community.adobe.com/t5/after-effects-beta-discussions/rotobrush-2-selection-expansion-issue/td-p/11641878

only, literally nothing will fix it. i've used rotobrush many times for years, on this same laptop/version of ae (completely updated)  and I have never had this issue before. it has always worked fine until now. im using the same settings on my footage that I always do. I tried everything suggested in the linked discussion, and nothing has fixed it so far. this happens when i try both rotobrush 2.0 and 3.0, and im not interested in using 1.0... any help would be greatly appreciated.