I was creating a frame-by-frame mask for a fairly complicated shot including two people moving. Luckily, the sequence wasn't long but I noticed that the mask I did few frames ago actually moved somehow in the meantime. For example, I adjusted the mask for frame 1 perfectly but by the time I was done with frame 5, the mask for frame 1 slightly but noticeably moved. Adjusting mask in subsequent frames shouldn't impact masking did in previous frames, or am I mistaken here? I don't know if it's of any relevance but I was adjusting and adding points to the 4-point polygon mask instead of using the free bezier mask. Additional question: Is there a way for me to "lock" the mask after each done frame to prevent it from moving? In the end, I had to give up because the mask kept moving no matter how many times I adjusted it.
P.S. A beginner here so even obvious answers could be helpful.