After I have managed to get the Prince teamed with a card costing up to 4, on my next turn it worked as expected. However, on the turn afterwards, the Prince and its companion had returned to my deck.
The card says that it must be "set aside" to remain in play, but I was not offered this option at the end of my turn.
Subsequently in the game I managed to get the same two cards paired, and this time the combination stayed in place for the remainder of the game.
So how should I have set aside the Prince the first time?
Game:
#70886023
Unfortunately, the game can't be loaded ("Internal Error"), so I can't look at what happened.
I could lookup the game's cards in Dombot, so I got a few theories:
1) Was the card you set aside with Prince "Walled Village"?
2) Are you sure that the Walled Village and Prince were returned to your deck?
Walled Village can in some circumstances be returned to your deck:
At the start of Clean-up, if you have this and no more than one other Action card in play, you may put this on top of your deck.
Normally, you are asked if you want to topdeck Walled Village if the conditon is met. Maybe you reflexively agreed to that. But there is also an autoplay that you can set to automatically topdeck the card without you getting asked.
This is what I suspect happened: The Walled Village returned to the deck automatically due to the autoplay, but Prince didn't (that's why I asked the second question). In this case, the autoplay was bad for you, and unfortunately the game doesn't recoginze this and does it anyway.
Of course, if you set aside another card, that theory is moot.
Excellent reply, and spot-on diagnosis: it _was_ the Walled Village.
I'll check my autoplay options.
Thank you.