Monitor loses HDR input on first connection after computer wakes

I have TBT3-UDZ connected to both a 16" Apple MacBook Pro and an LG 38GN950-B monitor. After the computer has been asleep and the monitor powered off, when I first turn on the monitor and wake the computer, the monitor’s image is dim, as if there’s a gray overlay atop the normal image.

If I toggle some setting on the monitor that causes it to reboot, such as changing the DisplayPort version from 1.4 to 1.2, or enabling or disabling overclocking, when it comes back the display is back to normal. It will stay this way until I put the computer to sleep again.

The monitor claims to be working at 75Hz with HDR enabled. I saw here that Plugable says this dock doesn’t support that refresh rate, but I didn’t choose it myself; it appears to be what the monitor negotiates with the computer on its own, just as it did with my previous Belkin Thunderbolt 3 Dock Pro.

Again, things appear to work fine after that first reboot of the monitor. What would cause the monitor to get into this “dim” state? I’ve used this same monitor and computer with two other Thunderbolt docks now, and neither exhibited this problem. Please advise.

I just noticed this: I disconnected the computer from the dock by pulling out the Thunderbolt cable from the MacBook Pro’s port. About a minute later, I reconnected the cable. The monitor came back on, looking good with its normal color, then, after about a second, went black, then came back in this “dim” mode.

Looking at the monitor’s reported status, it’s still running at 75Hz, but HDR is off. Looking in the macOS “Displays” System Preferences applet, the “High Dynamic Range” box was still checked. I toggled it off, and the monitor transitioned from that “dim” mode to its normal look. I toggled that box back on, and the monitor went black momentarily, then came back with its normal look. The monitor now reports that HDR is on again.

It seems that the Plugable dock has trouble starting things up with HDR active. In this case, the monitor was using HDR, got disconnected, got reconnected, and then lost HDR—until I toggled macOS’s use of HDR off and back on.


Thank you for contacting Plugable support! Sorry to hear about this issue. I’d be more than happy to assist you.

We’ve not encountered this issue before to my knowledge with this Thunderbolt 3 dock or our other models. I’m not sure what would be causing this rather strange behavior.

We have a troubleshooting tool that helps us find out what is going on in situations like this. Please plug the dock with your display attached into your computer, then go to this page and follow the instructions there:

I’d like to capture diagnostics when it’s in the bad state if possible. When emailing us the resulting diagnostics, please reference ticket # 326390.

Please don’t hesitate to let us know of other questions.

Thanks again for contacting Plugable support and best wishes!

Joshua Henry

Senior Engineer | Product Owner
Plugable Technologies

I upgraded my operating system to macOS Big Sur, to see if it would help. After the first boot into the new OS, the monitor worked as expected: When it woke up after being reconnected, it would go into HDR mode and stay there. However, after the first time that I put the computer to sleep and woke it again, the monitor returned to this problematic behavior of first attaining HDR mode, then going black for about two seconds, then coming back on without HDR mode, with that dim, gray overlay look.

I tried connecting the monitory to my previous Belkin dock again, and the problem does not arise. It’s definitely something wrong with the Plugable dock.

I thought that the problem could be due to the Plugable dock not supporting refresh rates above 60 Hz, so I used the “Displays” System Preferences applet to force the refresh rate down from 75 Hz to 60 Hz.

At first that seemed to help. The next waking up or two worked fine. By the third time, though, the dock was back to this same problematic behavior, disabling HDR after the monitor has been awake for about three seconds. The refresh rate isn’t the cause of the problem.

Thanks for these updates. Sorry for my delayed reply over the weekend, and to hear that nothing you’ve tried so far has helped. I’ve received the diagnostics you’ve sent over and am looking at them now for any clues.

We only officially support 60Hz, but on paper there is enough bandwidth for higher refresh rates for lower than 4K displays, and we do have several customers running higher refresh rates without issue. I’d agree that the 75Hz refresh rate isn’t causing this issue you’re experiencing.

To clarify, if HDR is disabled in the monitor settings, this issue doesn’t occur when waking from sleep, correct? If so, it may be a compatibility problem with HDR and our dock. I don’t believe that we’ve been testing any HDR displays, so this is a little bit of an uncharted territory for us.


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