There is a problem with Mono 6.0 where this doesn't work anymore for some reason. Try switching to linuxserver/radarr:5.14 tag and report again. The problem also occurs in Sonarr.
I have the same setup as you and it was working fine until mono 6.0 was introduced. https://www.reddit.com/r/radarr/comments/chp8tz/radarr_and_sonarr_copying_instead_of_moving/ https://forums.sonarr.tv/t/permission-issues-after-mono-6-upgrade/22876