Blacklist isn't working for flatpak mpv

Describe the bug
Adding "mpv media player" to the blacklist works for a given desktop session, but it appears again in the next.

To Reproduce

Play any media file with the mpv media player. (the flatpak version)
See that the player info is displayed.
Add mpv to the blacklist.
Player info disappears as expected.
Log out and log in again.
Check that mpv player is still on the blacklist.
Play any media file with the mpv media player.
See that the player info is displayed, even though it's on the blacklist already.
(Adding mpv to the blacklist again fixed it for the current session)

Expected behavior
Adding mpv to the blacklist works between sessions.

Screenshots
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Environment:

Additional info:
The blacklist works between restarts for the non-flatpak version.
For flatpak, it only works for the given session.

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