Did some more testing and googling and no idea what this could be, can't find anyone else complaining of something similar, I've been using snapshots together with send/receive to backup most of my Unraid servers for years now, most times during the backup I delete older snapshots from every data disk, and the space recovery always starts after a few seconds, in fact sometimes it even causes the server to perform slowly while it's deleting snapshots from multiple disks simultaneously, but it never took more than a few minutes to recover all the space.