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.