That's not a KDE specific issue, it's Qt defaulting to the default ugly old Plastic theme.

KDE uses Qt, but unlike GTK, Qt isn't made for KDE. Qt have their own defaults that are generic and doesn't make any assumption about running on KDE or Gnome. And Gnome doesn't bother providing any configurations to Qt, because they live in their own GTK bubble where nothing else matters. Adwaita just happens to be the default GTK theme regardless.

Ideally, your distro would take care of setting a saner default Qt theme. There's even a Breeze theme for GTK, so they could set it to that and everything would look nice and uniform.

Look into qt5ct, if you haven't already. That should allow you to retheme QT5 applications so that they behave as you choose even if KDE isn't running.

Isn't it more a problem of using something like QGnomePlatform and Adwaita-qt? AFAIK they cause a lot of issues and there are plans about discontinuing them in Fedora.

https://fedoraproject.org/wiki/Changes/NoCustomQtThemingForWorkstation

so they wouldn’t break completely when installed on the “wrong” desktop

the kde apps are made theme agnostics specifically to not interfere with the way other desktops/distro want to theme them, and the gnome desktop is specifically made to not natively handle user themes so as to not interfere with the way apps are supposed to look like, mix that into a bowl and you get ugly kde apps, which one is in the wrong is for you to decide*^1^, but at the end of the day you need another app to handle your qt-theming since gnome doesn't natively support it.