Usually if someone presses the key combination it is pressed like this (for example Alt+Z):
Push Alt -> Push Z -> Release Z -> Release Alt
And the last two actions are almost simultaneous. This is the reason why sometimes the combination is press in this way:
Push Alt -> Push Z -> Release Alt -> Release Z
In this second case Editor is losing focus and it is impossible to continue typing ... you need to press Esc to continue typing. And this is VERY inconvenient because it happens not regularly!

Try to press Alt+Z 10 times and see how many times you will end up in top menu selected (and editor will lose focus).

Newly created Qt Widget application does not have this issue ... so it is not about how Qt widgets behavior ... it is specific to Qt Creator.