I don't use Wayland yet personally but I imagine that the mpv ontop mechanism might be conflicting with the focus stealing prevention. Do you use ontop? Does it work if you disable ontop? If so, this should probably be considered a bug in mpv on Wayland.

hmm maybe not: https://github.com/mpv-player/mpv/issues/9064

edit: I just verified it is working fine on X11 (wanted to make sure I wasn't mis-remembering--but it is very obvious when something steals focus because I use the Dim Inactive desktop effect with a very high setting). I'm using ontop=yes as mentioned before. On Fedora 39 with my setup mpv does not steal focus when launching/switching videos. I did notice some weird behavior with nomachine before but after setting Focus Protection to Force High for nxplayer.bin I haven't noticed it again.

    KWin version: 5.27.11

If this isn't working in Wayland surely this should be considered a Wayland/mpv bug?--just one more reason for me to delay trying Wayland