I’ve had this same issue on Gentoo and now on Alpine, both with plasma 6 (Wayland). Pipewire and plasma 6 seem to be working as intended other than that… Any help would be appreciated!
E: the issue is plasma 6 exclusive, I have hyprland installed along side it and screensharing works just fine there
I’m pretty sure I had to enable the screencast use flag to get it to work for me.