Bugs related to this have been around for three years
The HDMI Forum has decided to respond to AMD's request for support for open source HDMI 2.1+ drivers. The answer turned out to be negative.
One of the limitations of the open source AMD graphics driver for Linux was the inability to implement HDMI 2.1+ functionality based on legal requirements from the HDMI Forum. AMD engineers worked with the HDMI Forum to develop a solution that would enable HDMI 2.1+ capabilities using an open source Linux kernel driver. Over the past three years, we have repeatedly heard about an error related to the unavailability of 4K resolution at 120 Hz when connecting to HDMI 2.1 in the AMD Linux driver. There were similar issues with 5K resolution at 240Hz.
AMD engineers worked with their legal team to evaluate all HDMI features to determine if they could somehow be implemented in an open source driver. AMD has been waiting for the last few months for approval from the HDMI Forum. Unfortunately, the request was rejected.
Unfortunately, the HDMI Forum rejected our proposal. Currently, an open source implementation of HDMI 2.1 is not possible without violating the HDMI Forum requirements.