I was finally able to reproduce this on stream. The frogs keep the head portion of the boss from spawning so the ground phases you cant be damaged. The delayed spawn of the boss waiting for the frogs to vanish more or less offsets it but its pretty neat.

I had that same thing happen without the frogs once but haven't been able to reproduce it :P But yeah thats so cool ^^

Nice find! I'd be curious to see if the corruption also happens on the arcade version. (Unlikely, but I might give it a shot.)

If I can find a consistent way to do it ill let you know, Ive only had it happen 3 or 4 times and only once without frogs, thought it was my cart at first. Curious if it has to do with the last mob dying to the fire orb in a certain spot too.

I think it once happened for me when i procced the fire orb while wyvern was off screen and it despawned the head then. Can't remember for sure if it was that but pretty sure it happened in those circumstances.