Yep, the detailed mode 2560x1440 is in the EDID returned by the monitor:
Standard Timings: none
Detailed mode: Clock 241.500 MHz, 598 mm x 336 mm
2560 2608 2640 2720 ( 48 32 80)
1440 1443 1448 1481 ( 3 5 33)
+hsync -vsync
VertFreq: 59.951 Hz, HorFreq: 88.787 kHz
Display Product Serial Number: GBLMQS047683
Display Range Limits
Monitor ranges (Bare Limits): 24-60 Hz V, 30-140 kHz H, max dotclock 300 MHz
Display Product Name: ROG PG278QR
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https://bugs.launchpad.net/bugs/1901470
Title:
resolution not listed in xrandr from ASUS ROG PG278QR via HDMI
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