Ok, that means that the issue is slimmed down to the permissions on the mounted smb share. Not docker or radarr which makes it easier to troubleshoot.

I am not a cifs share black belt though but continue from there :)

Can you create files and directories in /movies inside the docker container?

Have you set the correct chown permissions? ls -al /mnt/OrbiterVideo/movies

I have a similiar issue on a plex transcode folder that randomly resets permissions.

Does the ending / intervene maybe?

  - /mnt/OrbiterVideo/movies/:/movies

Is the share mounted?

Can you browse the mount from source machine?

Can you browse the share inside the docker container? (–interactive)

Or join a smaller instance