Broodshell Swarm boss bugged out, does it drop anything?

[Rattlesnark]

Does the boss (technically 'bosses', the two white worms + bell snail) drop any special reward on kill?

When I encountered this boss, I ran off far away to somewhere else. The boss's HP bar stayed at the top of the screen until at some point it started dropping for whatever reason. Eventually it hit 0 but remained on screen. I went back to the "arena" to have a look, and the bosses were still there chilling. I seemed to be able to kill them normally anyway but the empty HP bar UI remained there. Only way I got it to disappear was by exited to menu and loading back in.

I went ahead to collect the loot that was on the ground behind the arena, but I'm wondering if they have any special drops (like a tarstone) that they might have dropped if it hadn't bugged out.

EDIT: Guess I can answer my own question and leave it here for posterity.
Found someone's video on YT - the boss's HP bar does work a bit funky and there doesn't seem to be any on-kill drop. The reward tarstone is the one on the ground behind the arena.