Y'shtola triggers.
Y'shtola's effect resolves.
As part of the resolution, Watcher leaves, then enters again.
Both Watcher abilities go on the stack, you decide the order.
a) The "leaves ability" returns the card that was previously exiled to your hand.
b) The Hideaway ability exiles a new card.
No matter the order of the abilities, the newly exiled card does not return to your hand and if nothing else happens, the end result is the same.