Everytime, then you came back after setting mask, losing previuos setup or hard to change previuos setup. Something weird. I must save after every change, because there is no came back or if pressing undo, it shows wrong prieviuos setup or showing no mask at all. There is a bloody way to set this mask properly, but something is wrong there.
In better understanding, i imported 3 times same video to set mask about normal, because in 2 previuos i did UNDO, till point there was video imported, but mask still in place somehow. Can't find proper way to delete mask.