[btrfs-progs suspected BUG] Scrub info with Ubuntu-style subvol-as-root setup

Hi,

Setup: btrfs root but Ubuntu style: <btrfs root>/@/<root,etc,var,usr,..>
with root=@ as a kernel boot option. Been using it happily for years; not
even sure if Ubuntu still uses this system for btrfs installs.
btrfs fs is a partition on my GPT HDD.
My fstab contains subvol=@ as option on the btrfs line, and
adding/subtracting compress,autodefrag make no difference to this.

From Linux 4.4, I think, "btrfs scrub status" reports:

"scrub status for fdd6a335-6edf-****-****-****102cc8f5
	no stats available
	total bytes scrubbed: 0.00B with 0 errors" at all times including after
finishing.

I think it must be to do with the subvol mounted as root; pretty sure that 
The scrub appears to be doing something, but no info is available.

I haven't investigated much, it's not a show-stopper. I have waited until
4.4.1 btrfs-progs release before reporting, JIC.

Just to be clear: I've been using this setup for installs on everything from
Debian, Arch, Exherbo, CentOS, CoreOS (!), Gentoo, Funtoo .. without
problems. I've never actually used Ubuntu - knowingly!

Is there a heads-up about using this subvol-as-root setup I've missed? Does
anyone else use it? It's handy for rollbacks.

Thank you all.