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
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The KDE team has already determined that this is not a bug and that both you and me must just be imagining it:
https://bugs.kde.org/show_bug.cgi?id=481058
I mean, I get it being broken for the release candidate on gentoo... But it still being broken in the same way on 6.0.4 on alpine... That's gotta be one hell of an imagination