The HDMI Forum, responsible for the HDMI specification, continues to stonewall open source. Valve’s Steam Machine theoretically supports HDMI 2.1, but the mini-PC is software-limited to HDMI 2.0. As a result, more than 60 frames per second at 4K resolution are only possible with limitations.
Then it would be best to check the version of your distro and session, then go to their github issue list and search for it, if there isn’t one provide a issue for them to look at. I also recommend verifying if it’s not a hardware issue or cable fault.