Specs
Device: 2020 M1 Mac Mini, 8GB memory, 512GB storage
Monitor 1: DELL U3219Q, up to 3840x2160 resolution, via USB-C
Monitor 2: ThinkVision P27h-20, up to 2560x1440 res, via HDMI
The Problem
ThinkVision can display when roation is default (horizontal), however, after rotate 90 degrees, the highest resolution becomes 1344x756.
The Solution
Access the monitor settings in System Preference, where you can hold the Option button on the "Scaled" radiobutton and click, you should see more resolution options.
Before
After
A Bit Longer Story
I have a 13-inch Intel based Mac Pro laptop and a brand new 2020 M1 Mac Mini on my table.
Before I just use the 32-inch DELL monitor and connected my devices in this way:
Later I reorganized my table to create a better working from home environment, by which I added another 27-inch monitor to display documents and review code. It actually works fine, productivity improved, yay.
Then why not use two monitors after work with my own Mac mini? However, after I plug in another HDMI to connect the Think Vision with my Mac Mini, the issue happened. Bruh!
As for the reason, I don't know. Probably with the new mini configurations? It also has a couple annoying problems, such as bluetooth connection, or monitor won't turn on after restart if connected with a USB-C cable as the only monitor. I'm not gonna expand these topics here since I didn't really dive into the cause but hope above solution helps if you are having the same problem. Also this Ask Different forumn post maybe useful if holding the Option key doesn't work for you.