It doesn’t happen every time, but more often than not, when casting Mystical Mirage while playing Tombs of Terror run, since the last update, you can only play spells and weapons. Despite having enough mana, and not having any other effects that would disallow playing any minions, all of the newly created cards in your hand that are minions - are greyed out and can’t be played.
I just had the same problem.
Edit: I think that for some reason the bug only happens when you play it the first time and it has the twin spell keyword, usually the second use of it works normally, so I don’t know if the bug is tied to the keyword maybe?