I'm having issues setting up a mask path at around 2 second intervals. I set the mask and use a combination of curved and straight vertices then move on a further 2 seconds to my next keyframe. I then move the vertices for the new mask, changing some from straight to curved, curved to straight, add and remove new ones in some place to get the mask complete. And then I go on to do the rest.
Here's my problem. I find that the earlier vertices that I have set have changed in some (but not all) places, much to my annoyance and rework.
Editing vertex points on a mask can be troublesome to begin with. There's an option to "preserve constant vertex count when editing masks" in the preferences, but I'm not the one to explain that here. You might start googling about it. There seems to be a load of info on the net about it.