It seems like a bit of a tall order to expect this to work with a multi-device filesystem. To the kernel it looks like a second multipath connection to the same disk just appeared; how should it handle this? Refusing to mount the cloned block device looks sensible.
While not a multi-device filesystem, I believe XFS has the same behaviour and you have to change the UUID (or use xfs_copy in the first place) before you can mount a cloned block device.
(I wonder how ZFS handles this?)
Maybe it's not a code bug, but imposing global uniqueness requirements where you can't just `cp /dev/blkdev1 myfs.img` and be able to work with the new image seems rather like a design bug.