GNOME / GTK4 apps are developed primarily to work with mutter (GNOME’s wayland implementation) and few other prominent implementations (KWin for KDE etc), but not all.
GNOME is not the primary DE for RPi OS (though it should work), as RPi OS tries to run a minimal DE by default.
GTK / mutter devs should be able to give a more accurate explanation.
The real issue is, if you try to run an app developed for a particular DE in another DE, there might be issues and compromises. And this is one such example.
Having said that 'gnome-online-accounts' has now switched to the following dialog than a webkit webview for the 'Sign In' feature in version '3.49.0'.