Why corner pin isn't work with mask correctly?

I try to set my screen in new windows by CORNER PIN option. So i made it like this, but we can see still a shape of original screen:

When we use MASK now something strange is going one, becuse mask doesn't fit to the screen but to the "control points of original screen before using a CORNER PIN tool":

I can only make PRE-COMPOSE with NULL OBJECT and mask it again, then works

A question is: why we can't use a MASK on obcject modified by CORNER PIN?