The only real mystery here to me is why the helicopter pilot went from 200 ft, where they were suppose to be, to 350 ft. The plane pilot seemed to be flying the correct route and elevation. The ATC recording that was released was likely the pilot confirming visual with the plane that was taking off and was at a higher elevation. I don't think the helicopter pilot saw the plane that was landing, and I think they should have requested an elevation change in a high traffic area.
I also doubt the controller was watching that route at the time of the crash. It has been released that they were understaffed that evening, and that controller was operating two roles that night. I'd be willing to bet they were fulfilling their other job at the time and didn't know the helicopter pilot changed elevation.
Sure there are other things that need to be investigated like why they were understaffed that night, how often that happens, DEI hiring & standards, etc, but the elevation change of the helicopter seems to be what caused the crash. Things could have handle better by both the pilot and ATC.