Or you could right click top left corner of the window, More actions > keep above all others (make sure it is unchecked).
Should not be needed to make a window rule for this.
You can also include the button on most windows as default (looking at you firefox) via KDE settings: Appearence > Window decorations > make sure the “keep above other windows” is included in the window decoration.
When I open the application (whether from the minimised tray icon or from fully closing and opening the program itself) the window has ‘Keep Above Others’ checked. I can uncheck that and it will work, but after I close the program (either minimising it to tray or fully closing it) the next time I open the window it will be checked again.
To me that suggests I’ve set some default setting somewhere, but I’m not super familiar with KDE so I don’t know where to look. As far I can tell there are no Window Rules set:
Have you checked if there is any setting in the application?
Yakuake f.ex on x11 has a setting in the application doing this without a window rule (does not work on wayland though, there you HAVE to have a window rule if you want it to stay on top).
Scratch that, there’s a setting under the View menu tab for ‘Always on Top’ with the keyboard shortcut Ctrl+Shift+A, but the option isn’t listed in the Settings page. I must’ve activated that when mashing my keyboard.