I'm not sure what occurred in your experience, but I can tell you that we have successfully done a restore of a 10.3 backup to our 10.5 dev deployment with no issue.
That's what we were expecting. But for some reason it did not recognize we had 10.3 already installed. Once we saw it didn't recognize we cancelled. Possible the cancel did something. Not sure really.
I did talk with support a few moments ago and we can proceed with the install and do a restore from a 10.3 backup. Hope this helps anyone else.