Created attachment 126282 [details]
Gnome Calculator running side-by-side in Wayland and Xwayland modes

SUMMARY
GTK3 Apps in a Wayland session don't take on the Application Style assigned to them in System Settings > Application Style > Configure Gnome/GTK Application Style.  Instead, they take on a generic theme with pixelated fonts.  If I force them to run in Xwayland, the theme is correct.

STEPS TO REPRODUCE
1. Set a GTK theme in KDE System Settings.
2. Launch a GTK3 app in Wayland mode

OBSERVED RESULT
The app has a generic theme with pixelated fonts.

EXPECTED RESULT
The app matches the system theme.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 5.5.5-1-MANJARO
(available in About System)
KDE Plasma Version: 5.18.0, 5.18.1
KDE Frameworks Version: 5.67
Qt Version: 5.14.1

ADDITIONAL INFORMATION

This is a regression starting with Plasma 5.18. GTK apps correctly took on the configured theme in Plasma 5.17.x.