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.
3 monitors here and working fine on Elementary OS, Super+P works as well. Are you on Wayland or X11?
Whatever Cinnamon runs on natively. I have no idea beyond that.
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.
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.