Comment 4 for bug 1929456

I have decoded your monitor's EDID and it does show the desired resolution but only as a "Detailed mode". So that would appear to make this a duplicate of bug 1901470.

EDID version: 1.3
Manufacturer: GSM Model 23025 Serial Number 10845
Made in week 9 of 2015
Digital display
Maximum image size: 80 cm x 34 cm
Gamma: 2.20
DPMS levels: Standby Suspend Off
RGB color display
First detailed timing is preferred timing
Color Characteristics
  Red: 0.6513, 0.3320
  Green: 0.3066, 0.6308
  Blue: 0.1503, 0.0595
  White: 0.3134, 0.3291
Established Timings I & II
    720x400 70.082 Hz 9:5 31.467 kHz 28.320 MHz (IBM)
    640x480 59.940 Hz 4:3 31.469 kHz 25.175 MHz (DMT)
    640x480 75.000 Hz 4:3 37.500 kHz 31.500 MHz (DMT)
    800x600 60.317 Hz 4:3 37.879 kHz 40.000 MHz (DMT)
    800x600 75.000 Hz 4:3 46.875 kHz 49.500 MHz (DMT)
   1024x768 60.004 Hz 4:3 48.363 kHz 65.000 MHz (DMT)
   1024x768 75.029 Hz 4:3 60.023 kHz 78.750 MHz (DMT)
   1280x1024 75.025 Hz 5:4 79.976 kHz 135.000 MHz (DMT)
   1152x870 75.062 Hz 192:145 68.681 kHz 100.000 MHz (Apple)
Standard Timings
   1152x864 75.000 Hz 4:3 67.500 kHz 108.000 MHz (DMT)
   1280x1024 60.020 Hz 5:4 63.981 kHz 108.000 MHz (DMT)
   1280x720 60.000 Hz 16:9 45.000 kHz 74.250 MHz (DMT)
   1600x900 60.000 Hz 16:9 60.000 kHz 108.000 MHz (DMT, RB)
   1680x1050 59.954 Hz 16:10 65.290 kHz 146.250 MHz (DMT)
Detailed mode: Clock 185.580 MHz, 798 mm x 334 mm
               2560 2624 2688 2784 ( 64 64 96)
               1080 1083 1093 1111 ( 3 10 18)
               -hsync -vsync
               VertFreq: 60.000 Hz, HorFreq: 66.659 kHz
Detailed mode: Clock 148.500 MHz, 798 mm x 334 mm
               1920 2008 2052 2200 ( 88 44 148)
               1080 1084 1089 1125 ( 4 5 36)
               +hsync +vsync
               VertFreq: 60.000 Hz, HorFreq: 67.500 kHz
Display Product Name: LG ULTRAWIDE
Display Range Limits
  Monitor ranges (GTF): 56-75 Hz V, 30-90 kHz H, max dotclock 240 MHz
Has 1 extension block
Checksum: 0x7b

----------------

CTA-861 Extension Block Revision 3
Underscans PC formats by default
Basic audio support
Supports YCbCr 4:4:4
Supports YCbCr 4:2:2
1 native detailed modes
25 bytes of CTA data blocks
  Video Data Block
     1920x1080 60.000 Hz 16:9 67.500 kHz 148.500 MHz (VIC 16, native)
     1280x720 60.000 Hz 16:9 45.000 kHz 74.250 MHz (VIC 4)
      720x480 59.940 Hz 16:9 31.469 kHz 27.000 MHz (VIC 3)
     1920x1080 30.000 Hz 16:9 33.750 kHz 74.250 MHz (VIC 34)
     1920x1080i 50.000 Hz 16:9 28.125 kHz 74.250 MHz (VIC 20)
      720x576 50.000 Hz 16:9 31.250 kHz 27.000 MHz (VIC 18)
     1920x1080i 60.000 Hz 16:9 33.750 kHz 74.250 MHz (VIC 5)
     1920x1080 50.000 Hz 16:9 56.250 kHz 148.500 MHz (VIC 31)
      640x480 59.940 Hz 4:3 31.469 kHz 25.175 MHz (VIC 1)
     1280x720 50.000 Hz 16:9 37.500 kHz 74.250 MHz (VIC 19)
  Audio Data Block
    Linear PCM, max channels 2
      Supported sample rates (kHz): 48 44.1 32
      Supported sample sizes (bits): 24 20 16
  Speaker Allocation Data Block
    Speaker map:
      FL/FR - Front Left/Right
  Vendor-Specific Data Block, OUI 0x000c03 (HDMI)
    Source physical address 1.0.0.0
Detailed mode: Clock 148.500 MHz, 598 mm x 337 mm
               1920 2008 2052 2200 ( 88 44 148)
               1080 1084 1089 1125 ( 4 5 36)
               +hsync +vsync
               VertFreq: 60.000 Hz, HorFreq: 67.500 kHz
Detailed mode: Clock 74.250 MHz, 598 mm x 337 mm
               1920 2008 2052 2200 ( 88 44 148)
                540 542 547 562 ( 2 5 15)
               +hsync +vsync
               VertFreq: 60.000i Hz, HorFreq: 33.750 kHz
Detailed mode: Clock 74.250 MHz, 598 mm x 337 mm
               1280 1390 1430 1650 (110 40 220)
                720 725 730 750 ( 5 5 20)
               +hsync +vsync
               VertFreq: 60.000 Hz, HorFreq: 45.000 kHz
Detailed mode: Clock 27.000 MHz, 598 mm x 337 mm
                720 736 798 858 ( 16 62 60)
                480 489 495 525 ( 9 6 30)
               -hsync -vsync
               VertFreq: 59.940 Hz, HorFreq: 31.469 kHz
Detailed mode: 00 00 00 ff 00 35 30 39 4e 54 4b 46 30 41 38 34 35 0a .....509NTKF0A845.
Checksum: 0x8c