Unable to turn off HDR on external display after macOS 27.2 update

This may be a macOS 27.2 display-management issue involving HDR settings or the way macOS communicates with the ASUS XG32UCWG. 

Since you have already tested both USB-C and HDMI, I would recommend working through the following steps before assuming the monitor or cable is defective.

1. Check the macOS HDR setting

Go to Apple menu > System Settings > Displays, select the ASUS XG32UCWG, and look for the High Dynamic Range (HDR) option. Try switching it off.

If the toggle is missing, greyed out, or immediately switches itself back on, make a note of that behaviour. It could help identify whether macOS is failing to retain the setting.

Apple's documentation explains how HDR is managed on external displays:

Play HDR video on Mac - Apple Support

2. Completely power-cycle the monitor and Mac mini

• Shut down the Mac mini.
• Turn off the ASUS monitor.
• Disconnect the monitor's power cable for approximately 60 seconds.
• Disconnect the video cable from both devices.
• Reconnect the monitor's power and video cables.
• Turn on the monitor first, followed by the Mac mini.

This can force a fresh display handshake and allow macOS to renegotiate the monitor's supported display capabilities.

3. Check the ASUS monitor's HDR and picture settings

Open the ASUS monitor's on-screen display (OSD) menu using its physical controls. Check the HDR-related settings and picture modes. If an HDR-specific mode is selected, try switching to a standard SDR-compatible picture mode.

Also check whether any gaming, display enhancement, or automatic picture-mode features are affecting the monitor's behaviour.

ASUS support and product documentation:

https://www.asus.com/support/

You can search for the XG32UCWG to find the applicable user manual, firmware and support downloads.

4. Re-establish the display connection

Disconnect the monitor from the Mac mini and reconnect it directly, avoiding USB-C hubs, docks, adapters or KVM switches during testing.

After reconnecting, open System Settings > Displays. Hold down the Option key to reveal the Detect Displays option, if available, and select it.

Apple's external display troubleshooting guide provides additional instructions:

If your external display is dark or low resolution - Apple Support

5. Check for ASUS monitor firmware updates

Visit the ASUS support page and search for the XG32UCWG. Check whether ASUS has released a newer firmware version or any relevant compatibility updates.

https://www.asus.com/support/download-center/

Follow ASUS's firmware installation instructions carefully and do not interrupt the update process.

6. Check macOS updates

Go to System Settings > General > Software Update and check whether Apple has released a newer macOS update.

If this problem began immediately after installing macOS 27.2, it is worth considering a possible macOS display-management regression, particularly if the HDR setting cannot be disabled regardless of which connection type is used.

7. Report the issue to Apple

If the problem persists, submit a report  via the  Product Feedback - and make it known to Apple regarding this ongoing issue

Include the following information:

• Mac mini model and processor.
• macOS 27.2 build number.
• ASUS XG32UCWG monitor model.
• Both USB-C and HDMI connections have been tested.
• Whether the HDR toggle is missing, greyed out, or refuses to remain disabled.
• Whether restarting or power-cycling the monitor changes anything.

One additional question worth investigating: Does the monitor actually remain in HDR mode after you attempt to disable HDR in macOS, or does the macOS HDR toggle simply refuse to switch off?

That distinction is important because it can help determine whether the problem is with macOS retaining the display setting or with the monitor continuing to receive an HDR signal.

Apple's HDR troubleshooting documentation is particularly relevant here, as it specifically addresses disabling HDR on supported third-party external displays. ASUS's firmware and monitor documentation should also help rule out a monitor-side issue.

I have found a workaround: switch the refresh rate, and then HDR can be toggled off. I had never encountered this issue before macOS 27.2.

This does not solve your ASUS HDR issue, but I just tested the High Dynamic Range toggle in System Settings > Displays for my HDMI connected LG 32UN880-B display and HDR toggles just fine. This LG display supports USB-C DisplayPort but I have never used that connectivity option.

I use this premium Belkin HDMI cable between my M4 Mac Mini Pro (macOS 27.2) and the LG display.