Interesting. Could you post a link to the dongle?
https://www.anker.com/products/a8317?variant=42329259475094&...
Does your adapter work at 120Hz without updating the firmware? If it does, does it support HDCP?
Yeah I had to flash the firmware on the Anker dongle, following this guide: https://forums.macrumors.com/threads/dp-usb-c-thunderbolt-to...
I just tried streaming 4K through the Apple TV app on my M1 Macbook Pro and no issues, so I'm assuming HDCP works.