In a new support document, Apple has recognized that when an external monitor with ultra-wide or super-wide external connection is connected to a Mac with the M1 chip, some resolutions supported by the monitor may not be available at this time.
Apple says it is aware of this issue and has planned a solution for a future macOS update. Apple did not specify exactly when a fix will be available, but the first beta of macOS 11.2 was seeded last week, so that’s a possibility.
To see additional resolutions for your external monitor, click the Apple logo in the upper left corner of your Mac, open System Preferences, click Monitors, and press and hold the Option key while clicking “Scaled.”
There is an ongoing thread about this issue in the MacRumors forums.