Linguist does not use system window decorations

Qt Linguist seems to use some built-in window decoration, which is quite irritating on desktop systems that don't have these minimize functionality. See screenshot - left Qt 6.3.0 alpha / right Qt 5.15.8.

It seems like the window decoration comes from the assumption that we're on wayland (that is correct), but that should not affect the window decorations and make the application look like not being part of the party. Setting QT_QPA_PLATFORM=XCB when starting Linguist (or any other application compiled with Qt 6.3.0 alpha) makes it look as before.