Indeed it was the version of GNOME I am at. In a fresh Tumbleweed VM I see the Adwaita theme is applied to the mkvtoolnix-gui window. I'm gonna test adding Qt6 support to QGnomePlatform to see if the issue get fixed in the GNOME version I'm using daily because if the issue persists it means that GNOME 43 probably will break adwaita-qt and upstream will have to sort things out.