I had the exact same setup as the OP. One 6900XT worked fine, another didn't. It was a PowerColor Red Devil. Explained behavior, got immediate RMA. RMA card worked fine. It was one of the first produced and it's VBios was not happy with PCIe 5.
Bought this one 4 years ago next month just so I would have room and easy airflow. I run it with the front glass removed. 2 X 200mm fans in the front. Side fan blowholes going the way of the dino never made sense to me.