Excellent, so that makes sense as btrfs pools that are mounted by LXD will have the right mount options set automatically, but those that rely on a filesystem which you mounted yourself cannot.
In your case, you’re missing the user_subvol_rm_allowed mount option. Add this to your /etc/fstab or whatever mounts your btrfs filesystem in the first place and once that’s applied and visible in /proc/self/mountinfo you should be good to go as far as containers deleting subvolumes.