@chenhaha sadly no real progress. After communicating back and forth with apple, and providing packet captures, they still need more from me. Unfortunately, I left the company and lost the VPN access. Only if they had replied to me a little sooner...

There is a faster script to reproduce the leaking. I reported a deadlock issue on https://developer.apple.com/forums/thread/702174. It turns out that that script is also leaking IOGPUResource.