Thanks,

Looking at your xrandr.txt there is the modeline :

  1920x1080 (0xc6) 74.176MHz +HSync +VSync
        h: width  1920 start 2558 end 2602 total 2750 skew    0 clock  26.97KHz
        v: height 1080 start 1084 end 1089 total 1125           clock  23.98Hz

That means the refresh rate accepted by your display should be :
74.176MHz / (2750 x 1125) = 23.9760808081 Hz
Due to floating point error, in monitors.xml you get  23.976079940795898 Hz

Maybe the issue comes from this ?

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You received this bug notification because you are a member of Desktop
Packages, which is subscribed to gnome-control-center in Ubuntu.
https://bugs.launchpad.net/bugs/1892296

Title:
  gnome-control-center cannot change resolution of external monitor

Status in gnome-control-center package in Ubuntu:
  Confirmed
Status in mutter package in Ubuntu:
  Confirmed

Bug description:
  g-c-c is unable to change the resolution on my external 2560x1440
  monitor to 1920x1080 - when I try, the monitor turns off.

  However, the command "xrandr --output HDMI-1 --mode 1920x1080" does
  successfully change the resolution.

  xrandr for the monitor shows:

  HDMI-1 connected 2560x1440+0+0 (normal left inverted right x axis y axis) 
597mm x 336mm
     2560x1440     59.95*+
     1920x1080     60.00    50.00    59.94    30.00    25.00    24.00    29.97  
  23.98  
     1920x1080i    60.00    50.00    59.94  
     1600x1200     60.00  
     1280x1024     75.02    60.02  
     1280x960      60.00  
     1360x768      60.02  
     1280x720      60.00    50.00    59.94  
     1024x768      75.03    70.07    60.00  
     800x600       75.00    60.32  
     720x576       50.00  
     720x576i      50.00  
     720x480       60.00    59.94  
     720x480i      60.00    59.94  
     640x480       75.00    60.00    59.94  
     720x400       70.08  

  I tried changing the resolution to 1600x1200 using g-c-c and this did
  work.

  ProblemType: Bug
  DistroRelease: Ubuntu 20.04
  Package: gnome-control-center 1:3.36.4-0ubuntu1
  Uname: Linux 5.8.1-050801-generic x86_64
  ApportVersion: 2.20.11-0ubuntu27.8
  Architecture: amd64
  CasperMD5CheckResult: skip
  CurrentDesktop: ubuntu:GNOME
  Date: Thu Aug 20 11:28:08 2020
  InstallationDate: Installed on 2019-07-01 (415 days ago)
  InstallationMedia: Ubuntu 19.10 "Eoan Ermine" - Alpha amd64 (20190606)
  SourcePackage: gnome-control-center
  UpgradeStatus: Upgraded to focal on 2019-12-08 (255 days ago)

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