I had exactly the same bug yesterday evening.
We were playing against a kraken. I think it was a bot at first. The first time I noticed that something was bugged was when I saw bird signs from a place on the map were the kraken absolutely could not have been.
After we killed the kraken, its body just lay on the ground, exactly like in your video. The game wouldn’t end. When somebody on our team killed some random wildlife “hunter team wins” appeared and the game ended.
Afterwards in the lobby, a guy in the chat asked us whether we played against him as monster, as he wasn’t sure. He told us that he could not do anything. That he just followed the monster. Like he was his shadow. I think he said that he joined the game after it started.
Not much more info really but there is a pattern obviously. ^^