I’ve seen this twice against Expert Mage (might be single player only). I didn’t understand what happened the first time, but I paid more attention the second time. Playing Mistah Vistah into mirror entity seems to either freeze the game or restart it (I had one of each). Not sure if it’s because he’s a tourist or if it has something to do with the “Scenic Vista” effect.
This bug is accompanied by an invisible minion on the mage’s side of the board. I didn’t understand what was happening the first time – one of their minions was just pushed to the side. The second time, I understood it; there was a minion-sized gap between two of their enemies. Presumably this is the bugged Mistah Vistah.
Hypothesis: Maybe something like – the game is trying to grab the Scenic Vista for him when his owner casts a spell to store it, but it is null because it was never spawned (because he was copied and did not get a battle cry). I will try to pay more attention to what actually causes the hang/crash – see if it’s when the mage tries to cast their next spell for example.