[Dumytru Nemţuc]

Okay, I had the exact same problem. And it’s annoying!
And masking the previous frame wasn’t exactly an option, since in my case it would double the work I have to do.

So here’s what fixed it for me.
Context:
I have a lagarith video imported. In the “Interpret Footage” menu I selected “Use framerate from file (23.97)”.
I dropped the video to a new composition. As you probably expect, the new comp should have the same framerate as the footage. And it does, 23.97.
Solution: The actual framerate of the footage is 23.976.
In had to change it from 23.97 to 23.976 manually, both in the Interpret footage and in the composition, and that did it.

It’s likely that if you encounter this bug, it’s probably because of the incorrect framerate. After Effects apparently imported the footage with the wrong fps and when it rendered the movie, it used the actual fps, which messed up the masking.

Hope that helps some of you who have this very annoying problem.