That was not an error. If a card was to be banished at the end of the turn, and the end of the turn did not occur because of victory or defeat, the card should not be banished but instead be brought into the deckbuilding phase.
Sadly it was an error... this is relevant entry from Wrath of Righteous FAQ (the current valid rules set): "I displayed a card that tells me to banish it at the end of the turn, but I won the scenario so my turn never ends. I get to keep the card, right?Nice try, but no.
Resolution: On page 17 of the rulebook, under "Ending a Scenario, Adventure, or Adventure Path, replace "Once you’ve played a scenario, whether you won or lost, rebuild your character deck (see Between Games, below). Put all other cards back into the box" with the following:
After you complete a scenario, if any displayed boons could be banished or removed from the game when a certain condition is met (such as the end of an encounter, the end of a turn, or a location closing), treat them as if that condition is occurring. Then put all cards other than boons back in the box. Next, rebuild your character deck (see Between Games, below). Finally, put any remaining cards back in the box."