I'm trying my hand for the first time to do some masking on video footage. I have created my mask and keyframed it as necessary to correspond with the movement of the subject. However, I have found that in a couple of the frames (one of which, frame 109, is demonstrated in the screen capture below), the mask seems to "clip", not filling in the mask outline fully. Note that these frames do not have keyframes for the mask.
This does not appear to be a oneoff glitch. The clipping occurs in the same manner and on the same frames even when I re-open the file, and even if I render out the mask layer (in other words, it's not just a movie clip editor issue).
I've found that if I move certain control points on the mask slightly on the affected frame, the area will fill in correctly (but if I move them back to approximately where they were, it will "clip" again).
Does anyone know what is causing this or how to avoid it?
Note: This issue demonstrated in the screen capture below by setting the movie clip editor to mask mode and setting the mask display to Overlay/Alpha Channel.