In a new support document, Apple recognized that when an external ultra-wide or super-side monitor is connected to a Mac with the M1 chip, some resolutions supported by the monitor may not be available today.
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Apple says it is aware of this problem and has a solution planned for a future update of macOS. Apple did not specify exactly when a fix will be available, but the first beta for macOS 11.2 was sown last week, so that’s a possibility.
To view 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 a topic running on this issue on the MacRumors forums.