Anzrag is causing the game to crash when it dies and tries to destroy all of the gems. The same happens with Bovine Intervention, and basically any card that destroys all gems. Some gems, like the Guildpact Leyline are fine.
This crashes my phone, but not other devices like a laptop with an emulator. The crash costs me the match and health, just like quitting a match.
EDIT:
UPDATE: Midnight Snack did cause a crash in the Dusk Until Dawn event, during the upkeep, after it checked that I was at full life and tried to destroy all gems. Why it didn’t happen the turn before, when I put two of them on the field (and triggered the gem-wipe) I don’t know.
Cards that destroy all gems, such as Progenitus, Quake-Mole, and Bovine Intervention seem to cause the crash.
Cards that say they destroy all gems, but are tied to supports, such as Midnight Snack, or that skip loyalty gems like Scapeshift, don’t. It seems that if at least one gem is left, or the loyalty gems are skipped, the card won’t crash the game. I haven’t been able to more thoroughly test these cards by getting a lucky cascade to destroy all of the gems.