Qt Widgets | Strange QMdiArea's subwindow behaviour

Question

here is the problem: I have a menubar in my MainWindow. When adding a subwindow to QMdiArea and trying to maximize it, it's header, for some reason, idk why, sets on menubar. It looks like this: Click

Its too horrible, especially when u have, for example, a menu bar with a lot of sub menus. As a result, im trying to find solution to maximize window in boundaries of qmdiarea I would be very grateful if u provide me some solutions.

UPD: Also, is there any way to disable Qt icon, but not via setting a pixel (1,1) in it

I only found old topics where people said that this behavior is normal.. o_o