Could it be that automerge is not working because you are missing a hyphen? The movie section in the Jellyfin docs states:

To distinguish between versions, each filename needs to have a space, hyphen, space, and then a label.

https://jellyfin.org/docs/general/server/media/movies

Das ist wie mit der Bohrmaschine. Die muss auch ein paar mal durchgestartet werden, bevor es losgeht

I also use those, but (shame on me) I’ve never tested the backups. Have you ever restored such an image backup?