Co-op vs AI: everything is invulnerable

Description: Today I played an intermediate co-op vs AI game and roughly 5 minutes in, everyone and everything stopped taking damage, including champions on both teams, minions, jungle monsters, and turrets. It was funny for a bit, we eventually ff’ed because there wasn’t any way to end the game and everyone’s FPS kept dropping due to the massive number of minions.

Steps: not sure, other than load into an intermediate co-op vs AI game and play for a bit. Watched the replay but can’t pinpoint if anything unusual happened that triggered this behavior.