I am having this issue with Strawberry. Kvantum will not work with it. Apparently the app can't see it.

But all my other Flatpak apps can find the Kvantum style.

I try this in the terminal:

flatpak run --env=QT_STYLE_OVERRIDE=Kvantum --filesystem=xdg-config/Kvantum:ro org.strawberrymusicplayer.strawberry 

And I get this when the Strawberry starts:

09:34:19.914 WARN  unknown                          QApplication: invalid style override 'Kvantum' passed, ignoring it. 
09:34:19.914 WARN  unknown                              Available styles: Windows, Fusion 
09:34:19.928 DEBUG main:213                         Style: "fusion"

Is there a fix for this?