Poor HDR peak brightness. The Acer XV340CK can’t get bright enough to bring out highlights, so HDR content doesn’t look much different from SDR.
We measured the HDR peak brightness using the ‘HDR Mode’ Picture Mode, with Brightness set to max, and HDR set to ‘Auto’.
Note that there’s both an ‘HDR Mode’ Picture Mode, and an ‘HDR’ setting with an ‘Auto’ or ‘Off’ option. The ‘HDR Mode’ Picture Mode isn’t real HDR, as it only adjusts the colors to look like HDR. For proper HDR support, set HDR to ‘Auto’.