Thanks for the report, ehunt. This seems to be indeed a new bug that I haven't seen reported yet... EDIT:
Actually, it is correct. As you can see from what I wrote above, it took some time to realize that.
What happened is that this chain of cards played all originated from your Princed Vassal played at start of turn. That Princed Vassal played Golem, which played 2 Vassals... a.s.o. When you played that Prince that set aside Doctor, it was due to one Vassal in that chain.
So that Prince was played at a time that was sill considered "At start of your turn". When all the other effects of the chain had 'wound down' (there was one Vassal left, that only revealed a Silver), the game dutifully checked if there were more "At start of turn" effects, and there was: that Princed Doctor had not been handled yet, so that was done.