I came here to say that Imprisoned Felmaw is not random. I'm 100% sure.
I've had it hit face four times in a row, twice when the opponent had 3 minions on board, once when they had 4 minions and the other vs. a FULL board. I've also witnessed the same thing happening while watching streamers playing.
I've always suspected that Blizzard rigged the odds of certain cards, especially when it comes to "random" effects and interactions, but this time it's so blatant in your face that it's not even funny anymore.
Of course this is all impossible to prove, since we don't have access to the game's algorithm, but that is proof enough for me that there's nothing stopping them from rigging it. I'm not surprised.