Manjaro 21.3 (Arch linux) - Gnome 42.2 with Wayland
mpv 0.34.1-dirty mpv --version
mpv-mpris 0.7.1

Please, can you add fullscreen status signals (true/false) ? Like with vlc or celluloid :

dbus-monitor "type='signal',interface='org.freedesktop.DBus.Properties'" with celluloid :

signal time=1655305733.087620 sender=:1.155 -> destination=(null destination) serial=43 
path=/org/mpris/MediaPlayer2; interface=org.freedesktop.DBus.Properties; member=PropertiesChanged
   string "org.mpris.MediaPlayer2"
   array [
      dict entry(
         string "Fullscreen"
         variant             boolean true
      )
   ]
   array [
   ]

I get other signals, play, pause,.. but not this one.
I don't know if this is a mistake or if it has not been added.

Thank you for your work.
Yours sincerely.