DIAGNOSIS The USB-C input is likely defaulting to a limited color range (16-235) while your HDMI is set to full range (0-255). This crushes black and white detail, making everything look flat and washed out.
FIX First, on the monitor itself, open the OSD with the joystick. Go to Picture Adjust -> HDMI/Input Icon (make sure USB-C is selected) -> Black Level. Set it to 'High' for full range. Then, on your computer (macOS: System Prefs -> Displays -> Color; Windows: GPU control panel like NVIDIA Control Panel under 'Change resolution' -> Output color format), ensure the output is set to RGB, Full Range.
IF THAT DIDN'T WORK The USB-C port might be negotiating a different color space like YCbCr 4:2:0. Force RGB output from your laptop's display settings or GPU control panel. Also, make sure HDR is turned OFF in both the monitor's OSD and your OS settings unless you're specifically viewing HDR content.
HEADS UP The monitor has different picture mode settings per input. Your calibrated 'sRGB' or 'Custom' mode on HDMI doesn't automatically apply to the USB-C input. You have to set it up separately for that input source.