An Audience With The King

So I just did this fight for the first time this league. I went in, fought for a few seconds, phased the boss. Went in to the vine maze section, found my way back to the arena, started fighting the boss again. A few seconds later, I got the pop up notification that I completed the map and the challenge, but the boss was still alive (probably like 70% health if I had to guess?). Then the boss’ HP bar shot back up to full for a couple seconds, then the boss just died. I definitely do not do enough damage to have killed him that quickly. It was very fast, he didn’t even get to do any special attacks in the second round. Asked a few people if they’d ever seen that before, they agreed it must have been a bug.