Trial of Chaos Duo issues

I've been trying to help my dad get his 4th ascendancy through the Trials of Chaos, and we've been running into a couple issues, though this last one was an unfortunate bug.

We were running Trials of Chaos, and got to a "return the Soul Cores" room. I've been killing everything while my dad picks up the cores & returns them. All is fine until the last core, which he was carrying the entire time, when the game gave the message "Missing item for this interaction", while it's on his characters back, draining his HP. After he dies, I'm now able to place the Soul Core without ever picking it up, and without it draining my HP at all.

I hope they find ways to fix Co-Op in these ascendency trials, because Sehkemas is actually worse than Chaos is co-op, and the only way he's failed the Trial of Chaos with me is due to glitches such as this.