External HDMI Monitor Stops Working When AC Adapter Is Connected

[UMD]

Hello,
I have an issue with my HP Spectre x360 15-df1033dx running Windows 11.
My external 4K HDMI monitor works normally on battery, but goes completely black the moment I plug in the AC adapter. The laptop display continues working, but the external monitor never detects a signal.

Here are all the details and troubleshooting steps I have already taken:

Symptoms

• HDMI monitor works only on battery power

• The moment I plug in the AC charger → external monitor turns black instantly

• If I boot the laptop with AC connected → HDMI never activates

• The laptop charges normally

• The laptop display works normally

• Monitor works fine with other devices

What I’ve tried (none worked):

• Different original HP charger and a USB-C PD charger

• Different HDMI cable

• Direct HDMI connection (no adapters, no KVM)

• Hard EC reset (holding Power 15–20 seconds)

• Reset BIOS to defaults

• Updated BIOS to the latest version

• Fresh Intel GPU and NVIDIA MX250 drivers

• Disabled NVIDIA GPU completely

• Reinstalled all ACPI, power, and thermal drivers

• Reset Windows power plans

• Powercfg resets

• HDMI does NOT work even before Windows loads when AC is connected

Important Notes

• HDMI worked perfectly before

• The issue started suddenly after a normal reboot

• Monitor works instantly again if I unplug AC power

• Windows detects AC normally (battery icon changes)

My question

Is this a known issue with the HDMI or Display power rail / PD controller on the HP Spectre 15-df series?
And is there any official HP diagnostic or firmware procedure to confirm whether this is:

1. a firmware/EC/PD controller issue
   or

2. a hardware-level failure of the HDMI/Display power switching circuit?

Any guidance or official steps to diagnose or repair this would be very appreciated.

Thank you!

[UMD · Accepted solution]

The issue has been resolved. 

HDR was enabled for my external 4K monitor, but disabled on the internal laptop display.
When the laptop was running on AC power, Windows tried to switch both displays into a unified high-performance display pipeline, and the HDR/SDR mismatch caused the HDMI output to shut down immediately.
On battery, Windows stays in a lower-bandwidth mode, so the monitor worked normally.

After turning HDR off for the external monitor, the HDMI signal started working again with AC power and the problem disappeared completely.