I'm in my first playthrough in patch 2.1 and I'm not sure exactly what's causing it to happen, but it seems to be possible to refight the Frog King multiple times in the same playthrough. To cause it to happen, you need to beat the Frog King, then run in and out of the forest multiple times until you see the nu appear that says 'stop it you dog', and then wait for the nu to disappear from the map, not fight. When you enter the bush with frog in your party, it's as if you never beat the Frog King. I don't know if there's repercussions for beating the frog king a second time, so I just reset the game. Just thought I'd bring this glitch to your attention. I was puzzled as to why sometimes the event became undone and I realized that each time I triggered it was when the nu was present. I don't know if they share the same flag or what's happening.