Did you ever find a fix for this? This is like the 6th time I've tried the EE and like 4 of those 6 attempts, it gets bugged on the mimic blood step and no matter what I try I can't seem to get him to go into the chamber. I've only beaten the EE once with no bugs at all, the 2 other times I beat it, the elemental bow step bugged and the orbs wouldn't pop up, but I just shot a bunch around the general are that they are supposed to be in and it eventually worked lol.

So I noticed a similar bug when playing with another player instead of playing alone. The mimic blood step I BELIEVE can be solved currently by pressing the panel when the mimic is near the chamber, I know this is a visual bug because on SOLO gameplay the mimic automatically gets pulled into the chamber when near it but not with other players, i.e. the host. So just pressing the panel that you use to start the sequence was able to get me and my friend past the mimic blood step but when we did that we had the same bug with the ELEMENTAL ORBS where they wouldn't show up and we couldn't continue the EE.
I find it interesting the ORBS didn't visually show up at all but were still actually there in their respectable locations. I asked my friend (host) to check the ORB locations when I realized it might be a CLIENT side issue but he also didn't see the ORBS. Now that I know that the mimic blood step did actually INDEED work and the ORBS might have actually been there although we couldn't see them makes me think it could possibly be a Map Processing error with the zombie spawns because of how the zombies stop targeting the other players and only the host and their reduced spawn rates when the players are separated from the HOST PLAYER who would be on the surface or in the bunker. i.e opposite of other players. I appreciate you two for commenting your bugs and issues you encountered because we can actually probably fix this issue for the whole community if we just describe what happened in our situations.
<3 - J