Deserter Bug: Ability to steal mighty knight without fortress
The ability to steal a mighty knight without fortress (with the deserter card) is a bug in my view. It should work analogously to other cards/game rules, e.g. you cannot use medicine/wall/road card if you have already the maximum number of cities/walls/roads or analogously to the deserter itself, where you cannot place an additional single knight if you already two single knights or no place to put the knight.
So the stolen mighty knight should be degraded to the next available knight level.
I know that the following Catan FAQ link states that it is allowed to steal a mighty knight without fortress, but in my opinion it's wrong (the deserter card being one of the strongest cards already):
@phjma We moved the topic and are interested to hear what the community has to say about your proposal.
It's a game rule issue. I do not think you need to listen to what the community thinks of this. Ask directly from the game creators.