Just FYI, not sure if you’re intending to imply this but a lot of people get it wrong regardless and it’s worth saying: usb-c/thunderbolt was NEVER compatible with HDMI.
DP++ (the hdmi mode for DisplayPort connectors) is and has always been an optional extension, it’s just so widely supported on full-size DP ports that people don’t even realize it. And it’s NOT included in most embedded implementations of DP - like the usb-c alt mode. DP++ involves using a different voltage, and it would be super complex to do the voltage change in situations like that across a USB link... you'd probably have to have a voltage converter for the DP pairs to run at HDMI voltages.
Anyway, all usb-c to hdmi cables are active cables. The adapters for hdmi 1.4 are dirt cheap and they’re so small they fit inside the plug, but, they’re active converters and they can have all kinds of weird behavior. Same for docks, the hdmi coming off your thunderbolt dock is an active converter chip and I’ve personally experienced hdmi-specific glitches on a Dell thunderbolt dock that I think were attributable to this.