Because you guys used poison/ burn.
Using those when Gloopy at very low health won't kill him, but cause that bug, as you mentioned above.
Gloopy is not bugged. He is not designed to be fought like your typical boss. You need to use the environment around him, not fight him directly when he goes in to that state.
I've already explained above, Leo.
When you use burn/poison (especially long duration poison of raider) when Gloopy is at very low health (And he's in Exoskeleton mode when attacked with poison),
they won't kill him but cause him to glitch, to become immortal, unclickable and stay still.
In short:
- What causes the bug: Burn/ Poison (Especially Venom Knife which has long duration)
- When the bug happens: When Gloopy is at very low health, and in Exoskeleton, and attacked with burn/ poison
- What happens after the bug: After all globs are dead, Gloopy returns to normal state but stays still, unclickable.