In fact, despite the wording of the card text, Secret Passage does swap back at the end of the Rogue's turn. The reason the spells from Evocation are not discarded is because cards are always returned to their default state when moving between zones.
Specifically, Evocation creates a "discard at end of turn" effect that gets removed when the cards go into the "Set Aside Zone" created by Secret Passage.
Small (and admittedly inconsequential, pedantic, and unverifiable) correction to this. Cards dont change when moving between zones if they move between the zones in order. IE: Deck to Hand to Board. Cards only change back to default when moving zones out of order. IE: Going from board to deck via something like Psychic Scream. They've never clarified if this new "Set Aside Zone" is actually considered a zone, much less where it falls in the order. So there's no reason to think this interaction has anything to do with moving zones.
I believe what happens is the Evocation cards have a one-turn effect applied to them that says they're discarded at end-of-turn. However, effect triggers only happen when they are in the appropriate location during the check for the event trigger, end-of-turn effects are checked for while the cards are in the Secret Passage. Then they get re-added to hand but their effect was not put on the stack so the effect just fizzles.
In short, the change that stopped minions with "after a spell is played" effects from triggering when put on board via spells explains why the discard does not happen.