Since this happened three times in the same match with the same conditions I suspect there to be a bug. Here goes with the extremely specific conditions:
- Playing mobile Hearthstone on an iPhone 6S, iOS 12.2
- Playing the Dalaran Heist Adventure
- At least one copy of “The Darkness” dormant on the board each time this bug occurred
- Double Spell Cast Anomaly Mode active
- The “repeatable this turn” Hagatha Treasure which transforms the player’s minions into random legendaries on cast had its cost reduced to 0 for the entire run
- Whenever “Illidan Stormrage” was on the board for the first cast of the Hagatha treasure, Illidan’s effect would trigger, summoning a 2/1 token minion that would also be affected by the spell cast
- The game client would immediately crash seemingly before the first cast resolved, requiring the Hearthstone app to be restarted to resume
While I suspect most of these circumstances are irrelevant to any possible bug, hopefully this helps narrow down the problem should it exist beyond my own device.