Window space is reserved. You can check this by maximizing a window. If the window correctly fills the area under the taskbar then it is reserving the space correctly.
Unfortunately if an application has remembered the last location it was at and saved those coordinates then it may appear under a taskbar. This is why applications are supposed to save workspace coordinates rather than physical coordinates and also why setting absolute coordinates for a window to open at is incredibly bad practice.
Tests here on Windows 10 show the quick assist app sets absolute coordinates for a window to open (0,0) and will appear under the Windows 10 taskbar as well when at the top of the screen so this is not anything to do with Start11. I am afraid you would need to contact the manufacturer of that app for a fix for that.
If you have a window that is stuck, press Alt+space and then M and then use the keyboard arrow keys to move the window.
