It’s the multiple volumes that are throwing it.
You want to mount the drive at /media/HDD1:/media
or something like that and configure Radarr to use /media/movies
and /media/downloads
as it’s storage locations.
Hardlinks only work on the same volume, which technically they are, but the environment inside the container has no way of knowing that.
I wonder if they’ll replicate the feature where a strange voice whispers your name (amongst other odd sounds) if you’re playing in the early hours.
Scared the crap out of me when that happened.
Edit. Evidence https://www.ttlg.com/forums/showthread.php?t=6529 to show I’m not mad.