The Flatpak version of Stremio doesn't support using an external player.

[RadoslavL]

The option is just missing. I enabled the d-bus session and d-bus system permissions of Stremio to try to fix it, but that didn't change a thing. The logs don't say anything about mpv either. The Flatpak version seems to have its own version of mpv running internally, but it doesn't allow opening a seperate mpv window.

[eligamii]

Hi I got the same issue but I was able to fix it by doing this:
I installed flatpak-spawn, allowed dbus session permission and added the "org.freedesktop.Flatpak" as a Talks session bus with Flatseal

Then I modified the server.js file (at "/var/lib/flatpak/app/com.stremio.Stremio/current/active/files/opt/stremio/server.js") like this:

For any player in the "players" list (find it by searching var players = {):
Replace the "path" field with [ "/usr/bin/flatpak-spawn" ]
Add "--host", "\<the player you want to use>" before the other arguments of the "args" field

Then search for timeCmd + " " + argsCmd and replace it by argsCmd + " " + timeCmd

(by the way, sorry for my bad English)