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.



Last I heard Cinnamon does not have stable Wayland support yet. One of the reasons Wayland was made is because multi-screen support on the old X11 is an ugly hack. Unlike wayland, it doesn’t play well with screens of different resolutions, refresh rates, adaptive sync compatibility or HDR.
Oh, then it’s probably that issue.