Apple working on fix for ultra wide monitor issue with M1 Macs — Apple World Today

If you connect an ultrawide or super-ultrawide monitor to your Mac with Apple M1 chip, some resolutions supported by your display may not be available. Apple says it’s aware of this issue and resolution is planned for a future macOS Big Sur update.

To see the additional resolutions for your external display, choose Apple menu  > System Preferences, click Displays, then press and hold the Option key while you click Scaled.