The game “soft-crashes” with the message (“game completed and recorded! Restart Hearthstone to continue.” ). I noticed it happens if I add a Friend in the middle of the game or generally if I add friends (usually from previous games in regular Hearthstone).
This reddit reply supports the same exact reason: old.reddit dot com/r/hearthstone/comments/15c8irn/game_completed_and_recorded/juy7mbu/