So basically I've been having this problem in Vegas since I started masking but later I was just working on separate images rather than frames but it starts to piss me off when I'm about to mask like everything.
It sometimes happens that what is shown in the pan/crop doesn't fit what shows in the preview and later in the video. Like, the mask from the previous frame isn't showing there but on the next frame, where there is a new mask and the next frame is alright. So while I have 3 frames masked in pan/crop, in the video first frame doesn't have the mask, second frame has the mask from the first one, and third has the right one it should have.
What's weird, I have 3 scenes from the same video file - all of them masked. Two have this problem, one doesn't. I disabled resample on them and I checked if the frame rate in the project and the video file are the same.
Sorry if this has been asked before, didn't even know how to look for it.