External monitor blackouts with CoreDisplay errors after macOS Sequoia update on MacBook Pro

I originally described this issue in the Apple Community here: External Monitor Goes Black Randomly Afte… - Apple Community Other users are reporting similar symptoms after macOS 15.1.1, 15.3, and now 15.5.

This issue clearly began with the macOS 15.5 update in May 2025 and continues in 15.6.1. It is reproducible on M3 Pro MacBook Pro systems, affects multiple external monitors, and is tied to CoreDisplay and AmbientDisplayAgent errors. Logs and Safe Mode testing confirm this is not caused by third-party software or faulty hardware.

Since updating my MacBook Pro (Apple M3 Pro, 18GB RAM) to macOS Sequoia 15.5 in May 2025, my external monitor (Samsung Odyssey G5 LC27G55TQBUCEN / LC27G55TQWRXXU) randomly turns black for 20 to 40 seconds and then comes back on. This happens several times per day and continues on macOS 15.6.1. The problem did not exist before May 2025 on macOS 15.4. The monitor works perfectly with a Windows PC and a MacBook Air, so it is not a monitor or cable issue. This appears to be a CoreDisplay bug introduced in macOS 15.5.

Steps I have taken:

1.) Cables and Adapters

• Tested four different high-quality HDMI cables, all certified and purchased sealed.
• Tested two different DisplayPort cables, also certified and sealed.
• Tested multiple USB-C to HDMI dongles.
• Tested multiple USB-C to DisplayPort adapters.
• The issue occurs across all of these.

2.) Ports

• Tried both left and right USB-C/Thunderbolt ports.
• Tried both the upper and lower ports on each side.
• The issue occurs regardless of port used.

3.) Display Settings

• Set the monitor to 2560×1440 at 60 Hz.
• Tested at 1920×1080 at 60 Hz, but the issue still happens.
• Disabled HDR, Variable Refresh Rate, Adaptive Sync (FreeSync), Dynamic Brightness, Eco Saving, and Auto Power Off in macOS and on the monitor.
• Locked the monitor’s input to DisplayPort instead of Auto.

4.) System Troubleshooting

• Reset NVRAM/PRAM.
• Booted into Safe Mode (Boot Mode: Safe confirmed in System Report). The problem still happens.
• Updated to macOS 15.6.1. The problem continues.
• Before macOS 15.5, I was able to temporarily fix this by deleting the WindowServer display plist with:

sudo rm /Library/Preferences/com.apple.windowserver.displays.plist

After 15.5, this no longer works. I attempted to locate and clear all WindowServer display plist files in various locations, including:

/Library/Preferences/com.apple.windowserver.displays.plist  
/System/Volumes/Data/Library/Preferences/com.apple.windowserver.displays.plist  
~/Library/Preferences/ByHost/com.apple.windowserver.displays*.plist  
~/Library/Containers/*/Data/Library/Preferences/ByHost/com.apple.windowserver.displays*.plist

After restarting, the blackouts continued. This suggests macOS either moved or stopped using these files to manage external display stability.

5.) Cross-device Testing

• Same monitor and cables work flawlessly with a MacBook Air (not updated to Sequoia) and a Windows PC.
• Logs and Safe Mode testing confirm this is not caused by third-party software or faulty hardware. Safe Mode disables all third-party extensions and background processes, yet the monitor still blacked out. During blackout events the Mac continued to detect the monitor, and CoreDisplay logged repeated errors from AmbientDisplayAgent. This proves the connection itself is stable and the issue lies within macOS display handling.

During blackout events, repeated CoreDisplay errors are logged such as:

com.apple.AmbientDisplayAgent: (CoreDisplay) [ERROR] - Unable to create and lookup port "com.apple.CoreDisplay.master" => 1102
CoreDisplay_IsCustomWhitePointSet
getCustomWhitePointXYZ
modifyAndSetIntentionForWhitePointOnEveryDevice

There are no EDID, timing, or link retrain errors, which suggests the connection is not being lost. The display remains detected by macOS while the panel goes black. The external monitor should remain stable at 2560×1440 at 60 Hz, but it intermittently blacks out and then reconnects automatically. The problem is consistent across HDMI and DisplayPort, multiple adapters, multiple cables, and all MacBook Pro ports.

I hope Apple engineers review this closely, because the number of users reporting similar symptoms shows it is not an isolated case. This has disrupted my daily work and productivity since May 2025. A fix in a future macOS update would be greatly appreciated. In the meantime, I am sharing these details so that others experiencing the same issue know they are not alone and do not waste time chasing cable, port, or monitor replacements.

[Re-Titled by Moderator]

Original Title: CoreDisplay Bug? External Monitor Blacks Out Randomly After macOS Sequoia 15.5 Update (MacBook Pro M3 Pro, Samsung Odyssey G5, DisplayPort/HDMI)