To be honest, i lost track of the number of times I thought that, spent some time fiddling with detecting windows and could never solve any annoyance ever with window rules.
If I could have gimp menus as permanent “tool” blocks on the side bars I would be a happy gimper… But out of all the investigation from gus77, that the windows are actually “menus”… I would say:
kwin have a bug of not placing all “menus” belonging to an application, on top of said application when it is moved to the front.
(IMO this is the most obvious and expected behaviour, and i cannot think of any argument against it no matter how contrarian I try to be. I don’t even think it requires special type translations as suggested. Jut treat as a menu and behave as a menu.)