This is unimportant and may be related to KWin's signals. I report it just not to forget it; might close it later.

On opening each new window, the current desktop button blinks once under kwin_wayland — it's probably unchecked and checked again instantly, although I can't be sure, because it happens in a fraction of a second. There may be more conditions for seeing it though.

Franky, I don't remember if this happened on kwin_x11 too — have no motivation to log into X11 either.