I have recently started playing a lot of coop having mainly played in solo thus far. Every run it seems the connected players do not see the telegraphed AOEs on the floor for bosses.
We fought Bug Momma a few times and had the issue every time and the connected players would die in her breath/sick attack or her circular floor AoEs as they could not see where they would be.
We also had the same thing on other bosses but we mainly played Bug Momma as a friend needed to kill it for Cartie's quest. It was however impossible to attempt without everyone apart from the hist dying to invisible AoE.
It will not let me upload player.log as when I do it returns a filesize error!
In multiplayer, a client player may not see red triangle warnings before Bug Momma's AoE attack. The animation itself can be desynced as well so she looks idle when the attack hits.