Contagion Engine bug
I had Contagion Engine in play, which put the -1/-1 counters onto the opponent's creatures correctly. On their turn, they cast Requisition Raid to destroy it, which took away the -1/-1 counters when it left the battlefield.
(And I lost because of it! 6-3 in draft... so close)
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Crwth#63253 commented
Ah yes. Because that last mode happens instantly (no targeting effects to show that it's happening), I must have missed that. Thank you. (Had I gotten priority when it was cast, I might have noticed the third mode highlighted, but I was all tapped out after casting the Engine, so no priority)
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Rezzahan#77802 commented
the third mode of Requisition Raid puts a +1/+1 counter on all their creatures. +1/+1 counters and -1/-1 counters annihilate each other as a state based action.