STEPS TO REPRODUCE
1. Add bottom Panel with "Always Visible" visibility option;
2. Launch Wine application that forces fullscreen resolution (in my case - HoMM3 under wine-ge-8-25);
3. Try to do some actions (in my case - click some in-game buttons).

OBSERVED RESULT
Fullscreen window shrinks making free space on top and in place of the panel, then returns to its original position.

EXPECTED RESULT
Nothing like this.

SOFTWARE/OS VERSIONS

KDE Plasma Version: 5.114.0
KDE Frameworks Version: 5.114.0 
Qt Version: 5.15.12
Kernel Version: 6.7.1-arch1-1 (64-bit)

ADDITIONAL INFORMATION

Tested both on X11 and Wayland sessions.

Observed on:
- HotA (HoMM3 + HD) under Wine while interact with some action buttons;
- Path of Exile under Steam's Proton Experimental while interact with options menu.

Seems like this effect doesn't appears when Panel has "Auto Hide" or "Windows Can Cover" visibility option.