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My problem is similar to Macbook Air 2019 - External 4K monitor loses resolution. I have a brand new (one month old) MacBook Pro M1 running Big Sur with the latest updates. I have an Acer EB321HQU external monitor connected via DisplayPort to a j5Create USB-C hub, and I normally use it in clamshell mode. I have Battery preferences set to put the display to sleep when idle, not sleep the computer.

I set the screen resolution to the highest available: 2560x1440. This works fine most of the time, but every now and then (3 times so far) when I wake it, the resolution will be extremely low, like 800x600. When I open the Display preferences, the maximum resolution listed (even when I use the Option key and click on "Scaled") will be 1920x1080.

Eventually I do all sorts of fiddling around and the high resolution settings reappear, but it seems really random. When this happened a few days ago, it switching between color profiles seemed to do it, but that didn't work today. This time, I put the display to sleep manually with Cmd-Control-Power, and when I woke it again the higher resolutions were back.

As far as I know, I hadn't done anything unusual before it went to sleep. It just seems to happen totally randomly.

Anyone know what could be triggering this, and a more reliable way to get the high resolution back?

In another question about monitor resolutions, someone mentioned Display Maestro. I've just downloaded the trial version, so I'll give it a try if the problem happens again. But I'll still post this question now in case someone has more insight into the problem.

EDIT: It happened again today, and Display Maestro didn't help. When I opened the laptop lid, it had spontaneously unchecked Mirror Displays mode. While in non-mirror mode I could see 2560x1440 on the external monitor's scaling options, but when I closed it they went away. I fiddle for a while with different modes, then put the display to sleep again and woke it up, and then I was able to mirror it in high resolution.

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  • And if you plug the monitor directly into the laptop? Sep 26, 2021 at 20:12
  • I don't have a USB-C to DisplayPort adapter (or adapters for any of the other inputs of the monitor).
    – Barmar
    Sep 26, 2021 at 20:26
  • It's not an adapter, it's just a cable, such as amazon.com/gp/product/B01J6DT070 Sep 26, 2021 at 20:29
  • But I don't have anything like it. Also, it wouldn't be feasible to use it regularly, the laptop only has two ports. One is for power, the other connects to the hub, so everything else needs to use the hub.
    – Barmar
    Sep 26, 2021 at 20:36
  • I don't see any devices on j5Create website that are DisplayPort... doesn't mean there isn't one. But they have several that provide power to the laptop, you should use something like that. Sep 26, 2021 at 23:05

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I still don't know why this is happening, but I've found that I can now reliably fix it by simply manually putting the display back to sleep (with Control-Shift-Power) and then immediately waking it again.

UPDATE:

That wasn't always quite as reliable as I thought, sometimes I had to do it twice.

However, the problem has gone away completely since upgrading to Monterey.

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