TLDR, in the Expert difficulty of the Kell's Fall exotic mission, the Illusory Anchors that spawn for the Revenant Slayer I, II, III, and IV quests do not register progress for all players in the fireteam when shot by one player, and shooting one while in a fireteam can cause other players to be unable to see/shoot that Anchor unless the mission is relaunched, or a hard wipe is triggered.