Basically, when me and my friends kill a boss, it doesn't counts for the world. For example you know when you kill Plantera, usually there's a message after killing for the first time, but we don't have that. When we check through the boss checklist, they're not checked either. As if we didn't kill the boss.
When i switch to singleplayer, and then kill the boss and check, it counts.
Anybody knows which mods could be causing this? If needed, i could send my modlist, but i'm curious which mods in general could cause that kind of problem.