Masking bug in Premiere Pro (MAC)

[Liu26199853musa]

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It happens several times when I finished editing the mask and come back to adjust a few.

When I checked out frame by frame, the mask starts crazy which it doesnt match what my original position have put in, and then I started to move it around and readjust to the position I wanted, the masking failed to follow my instruction as shown in the picture (the masking is worked but the position has gone crazy), it is annoying and I cant find any help or bug issues on the internet.... I wanna know whether it is my problem/ wrong setting or any else (p.s. I upgraded my mac computer few months ago, so it is supposed to handle this basic masking effect)