Thanks. That indeed confirms the 99.98Hz mode has suddenly gone missing
in the last attachment. The system seems to be treating that as a
different monitor configuration and will revert to defaults (which is
landscape and 59.97Hz).

When this happens though, the monitor's EDID does not change. It still
reports the 99.98Hz mode exists:

Detailed mode: Clock 543.500 MHz, 797 mm x 333 mm
               3440 3488 3520 3600 ( 48  32  80)
               1440 1443 1453 1510 (  3  10  57)
               +hsync -vsync
               VertFreq: 99.982 Hz, HorFreq: 150.972 kHz

So if the mode exists but is not offered as an option then that suggests
the kernel or driver thinks there's not enough bandwidth or a good
enough connection to support that particular mode. Therefore, please try
a different monitor cable.

** Summary changed:

- Display orientation resets on reboot with USB-C (DisplayPort) external 
monitor and refresh rate dropdown disappears when setting it sometimes
+ 99.98Hz display mode is sometimes missing on USB-C (DisplayPort) external 
monitor, which resets the display layout

** Package changed: gnome-shell (Ubuntu) => linux (Ubuntu)

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https://bugs.launchpad.net/bugs/1922540

Title:
  99.98Hz display mode is sometimes missing on USB-C (DisplayPort)
  external monitor, which resets the display layout

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