@shocker Welcome to the Rockstor community and thanks for reporting you findings on this one. Yes the system on mdraid was purely to address redundancy and as you say, as far as btrfs is concerned this is a single device install. That way we circumvent the current issues with booting btrfs directly. Bit of a palaver to install this way but it does seem to work in the end.
Linking to @Spectre694 's thread “Install to full disk BTRFS” which has an informative discussion in the area of booting to btrfs.