In Flatpak apps, audio output switching does not work with WirePlumber

On Fedora 35 beta, with WirePlumber:

EasyEffects from a Flatpak is not working due to audio outputs not getting rerouted through EasyEffects
Spotify, also in a Flatpak, does not switch audio outputs from speakers to headphones or vice versa (an app restart is now needed)

Notes: In Fedora 34, Spotify worked as expected and apparently EasyEffects from a Flatpak does too. In Fedora 35, EasyEffects works when installed as an RPM. So this is a UX regression from a non-WirePlumber system and seems to affect Flatpaks specifically.

Issue for EasyEffects @ https://github.com/wwmm/easyeffects/issues/1144; this comment is specifically useful (with video demos as well): https://github.com/wwmm/easyeffects/issues/1144#issuecomment-930761301