Mac OS resetting monitors to 40-60Hz variable refresh rate after reboot or sleep, sometimes also swaps left/right display.

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Level 1

40 points

Mac OS resetting monitors to 40-60Hz variable refresh rate after reboot or sleep, sometimes also swaps left/right display.

I have two BenQ EW3270U monitors connected to my Mac Mini (Late 2018) via an eGPU (Razor Core X eGPU with AMD RX 6600 XT) using DisplayPort cables from the eGPU to the monitors.

Every time I reboot my computer or wake it from sleep, the monitor refresh rate changes to 40-60Hz variable refresh rate. I want to use the supported 60Hz, as variable causes flickering when I move things around in Finder or Photoshop. I would also like the right monitor to always be the right monitor, but Mac OS often swaps them around on wake/reboot, so I have to move the cursor out of the right edge to get to the left monitor and vice-versa. I then have to spend time arranging them and setting the refresh rate in system prefs.

I realize there are several factors that can cause this:

The monitors are the same model, so Mac OS confuses left and right on wake. However, Mac OS can actually see the serial numbers in the System Information panel, so my Mac could easily tell which is which, if Apple had just programmed Displays Preferences so it identified each monitor using their unique serial numbers instead of the brand/model.
My monitors are set to turn off after 10 minutes, not just sleep. Why? I'm conscious of my electricity bill and the environment.
The monitors support AMD Freesync and the AMD eGPU seems to think I want to use this. It's easy enough to blame the monitor manufacturer, or bad AMD-drivers (Apple no longer cares about eGPUs since they are not compatible with Apple Silicon) or even claim that a slightly off-spec GPU card is the problem, but if Mac OS simply saved my settings, and sent the "Monitor with SN#A is left, Monitor with SN#B is right, use 60Hz" signal on wake or startup, this issue would be moot. So that's a lazy excuse. If it can be adjusted via Mac OS prefs, then it can also be saved and sent on wake or startup.
I've been digging into this issue a lot, and it also affects Apple Silicon computers using two identical monitors, especially if both are using USB-C and none re using HDMI. The reason users don't want use one HDMI and one USB-C port, is the color output is different on each display port. And with a powerful eGPU, I don't want one monitor to run on the internal GPU, as it's very weak compared to the Radeon RX 6600 XT.

In conclusion, the one-size-fits-all solution would be if Mac OS identifies each unique monitor correctly using the transmitted serial numbers, and actually saved and applied the user's preferred Hz settings.

I have even tried using "Displaycer" and while it solves the left/right settings by identifying each monitor by serial number (see? it can be done) it fails to set the Hz.

Mac mini

Posted on Apr 24, 2025 5:40 AM

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Answer 1

Old Toad

Level 10

215,388 points

Apr 24, 2025 3:26 PM in response to The Heftster

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following;

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

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Answer 2

den.thed

Level 10

91,012 points

Apr 24, 2025 4:02 PM in response to The Heftster

The 2018 Mac mini is more than capably of running both of those displays without the use of an eGPU.

see "Video Support"at > Mac mini (2018) - Technical Specifications - Apple Support

This swapping and setup loss that you are experiencing, also occurs if both displays are connected to one of the Mac mini's ports using a Hub, Dock or Daisy chain setup. I personally do not have an eGPU to test.

What I would do first, is test the setup with one display connected to the eGPU and one display connected directly to the Mac mini using a short high speed USB-C to DisplayPort cable.

for example > https://www.amazon.com/dp/B0CKSVGJP5/ref=sspa_dk_detail_4?

Then if that doesn't help, I would temporally ditch the eGPU and test the Mac mini with both displays staggered on the TB/USB-C ports using two short high speed USB-C to DisplayPort cables.

for example > https://www.amazon.com/dp/B0CKSVGJP5/ref=sspa_dk_detail_4?

The other option is to test one display on a short high speed USB-C to DisplayPort cable and the other on a short high speed HDMI to HDMI cable.

for example > https://www.amazon.com/dp/B08TGBLLXY/ref=sspa_dk_detail_1?

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Answer 3

The Heftster Author

Level 1

40 points

May 1, 2025 12:39 AM in response to den.thed

Yes it's 'capable' of 'running' both displays with an eGPU. But the actual performance is ... let's say... impressively unimpressive. I use my Mac Mini for multimedia production, so 4K video editing, poster design in Photoshop with dozens of layers, complex vector graphics, layout for print etc...

I'm not attaching the displays to the internal Thunderbolt ports. It might help with the refresh rate, yes, because the Mac sees the displays as being different based on them being attached to diffrent connections. It would probably stop resetting the refresh rate on one of them, maybe even both. But I need that eGPU.

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