I asked the question OMV 2 versus OMV 3 - Growing RAID 5, in a new thread. Here's to hoping one of the programmers, a moderator, or other subject matter expert takes a look.
Wow took sometime for some replies. I will give your link a shot, but yes, I think something is not working right in OMV3 when it comes to growing a RAID5. Yes, I did wipe the drive prior to hitting the button.
Update - your command to add the spare to the RAID in your link worked! Reshaping now!