The HDMI port on newer Macs doesn’t work with DDC, I’ve tried to document that here: https://lunar.fyi/faq#brightness-not-changing

    These HDMI ports use an internal MCDP29xx chip to convert the HDMI signal to DisplayPort. That chip supports DDC, but the method we use to write the DDC commands to the monitor (IOAVServiceWriteI2C) fails when used through it.
    
    There are other WriteI2C methods specifically for this chip inside the DCPAVFamilyProxy.kext but they’re not available to be used inside an app like Lunar.

Asahi Linux shouldn’t be limited by the API, but the problem might be in the MCDP chip firmware itself. 1 year later, I’m still investigating new methods to fix this but so far I have no results.