"If undefeated... add a random monster to a random open location deck".
Is that "shuffle" or "put on top"?
This is from RotR, but I'm pretty sure it's still in there.
If you’re instructed to add a card to the top or the bottom of a deck, do so; otherwise, any cards added to a deck are shuffled into it.
So, since it doesn't say top or bottom, it's shuffled.
The same text is on page 15 in the other three rulebooks.