Thanks to @grebnek in the following forum thread for reporting this bug.
https://forum.rockstor.com/t/finally-build-my-own-nas/1811

Steps to reproduce (Rockstor version 3.8-14.07):-

Had a single disk pool with single raid level.

Resize Pool - Add Disk - Yes to change RAID level - selected raid1 from drop down and ticked additional disk - Next

Waited until balance complete.

Pools - rock-pool - indicates "Raid Configuration: single"
as does the pools column in the Pools page overview.

Expected result = indication of raid1 for this pool.

Confirmation of correct balance from single to raid1:

btrfs fi df /mnt2/rock-pool/
Data, RAID1: total=112.00GiB, used=110.03GiB
Data, single: total=1.00GiB, used=0.00B
System, RAID1: total=32.00MiB, used=48.00KiB
Metadata, RAID1: total=1.00GiB, used=390.86MiB
GlobalReserve, single: total=144.00MiB, used=0.00B

This replicates @grebnek findings.