On ipad, double tapping anywhere on the toolbars makes cubasis full screen/windowed.
I know we shouldn’t be double tapping the toolbars. But it happens often.
Or is it an iPadOS feature and it cant be disabled?
Hello and welcome,
you have to disable ios26 window mode and switch to fullscreen app mode in setting multitouch gestures.. 
It´s not from cubasis 3..
It happens often to me too.. 
It´s like the double tap to fullscreen window in macOS and windows..