Not sure how the mpris libraries acts here, but I guess that something like is done for libappindicator and libunity (more recently) is done [1].
So basically when in snap context, the app should prepend the desktop-id with its snap namespace when looking for its own desktop-id.
[1] https://code.launchpad.net/~3v1n0/libunity/desktop-use-snap-namespace/+merge/337252
Not sure how the mpris libraries acts here, but I guess that something like is done for libappindicator and libunity (more recently) is done [1].
So basically when in snap context, the app should prepend the desktop-id with its snap namespace when looking for its own desktop-id.
[1] https:/ /code.launchpad .net/~3v1n0/ libunity/ desktop- use-snap- namespace/ +merge/ 337252