I guess it must be something related to the PMDG side on the way their pushback works. Because when I hit shift P for pushback it does the same thing so I don’t know. Learn to live with it I guess.
Well, you shouldn't normally have to Stop the pushback before its regular end, that's usually more like an emergency feature rather than the norm.
If you use the automatic Left/Right pushback, it will stop automatically on a point on the taxiway line where the airplane and the tow truck are aligned, and if you use QuickEdit, it will stop exactly where you told it to.