Frostmourne killed 3 minions, at 16, 18, and 20. board was empty so there was space for all 3 minions to summon but the deathrattle did not activate.
hs replay: replay/ykYxWDm3kBeJ3dNwHUmTsb
I just had the same thing happen!
hs replay replay/SP6NhLZY8tm6NaeBu3wKv4
The same thing just happened to me. Played a ranked wild game. Killed 3 minions with Frostmourne, including a Sylvanas, a Thaurissan, and one of those [2/6, taunt deathrattle: destroy a random enemy minion] priest cards. Nothing was summoned with the Frostmourne deathrattle, and I lost the game because of it. Not cool!
Same Thing just happened to me, and I made sure that I did kill every minions with the weapon. animation of frostmourne happens, but nothing gets summoned. Almost lost because of it
Pretty major bug. Still happening. Just used Frostmourne to kill 3 minions in the final boss of Chapter 3, Heroic. Animation happens and no minions are summoned.