if you change your hdmi sound output profile to “pro audio”
plasma gets confused and the hdmi audio device disappears from the sound applet and settings – it stays like that even after reboot. as a gentoo user pointed out this setting was stored here:
getting rid of the wireplumber config dir (or just mentioned file) should do the trick if you are in the same situation. and remember not to do again what’s shown in the screenshot above!
since this happens on two different distributions it appears to be an upstream issue.