vivid heron
Arbitration doesn't count as killed because you're killing it on a subworld, which, well, is another world. That's also why the community's progress is set back, since apparently cal bosses don't sync when travelling to subworlds.
Pretty sure there's a mod that solves that issue, though.
heady cape
https://youtu.be/C1_gU4K9zF8 (Video of what I mean)
I do have the unoffical patch, not sure if that is what mod you are referring to.
vivid heron
Multiplayer unofficial patch? That's a different thing