OK I just did a fresh copy of one of my existing containers to the target.
Then went to delete it on the target and got the exact same BTRFS error as when I do a refresh copy.
lxc delete container
Error: Error deleting storage volume: Failed setting subvolume writable "/var/snap/lxd/common/lxd/storage-pools/default/containers/container": Failed to run: bt rfs property set -ts /var/snap/lxd/common/lxd/storage-pools/default/containers/container ro false: ERROR: Could not get subvolume flags: Invalid argument
So it is 100% related to this topic:
I will jump over and follow up on that topic so this one can be closed.