I have a 4 drive btrfs RAID10. In the webUI, in File Systems, in the Device column it is listed as "/dev/sdb<br/>/dev/sdc<br/>/dev/sdd<br/>/dev/sde".
It mounts and is accessible to the system and seems to be working fine. Just is listed oddly.
Also, while on that screen and watching 'top', I see 'btrfs fi show .....' keep coming up (4 at a time) in top. If I log out of the WebUI, that stops coming up. That may be by design so it can update the page with any new information, but it does consume more CPU time than I would like to think.