You probably have to update the monitor info DB on MacOS. See https://apple.stackexchange.com/a/233854
Re:Time to Upgrade Your Monitor
by CaptainMarvel on Thursday June 18, 2020 @08:03AM
(#23562248)
Text looks similar to Windows and Linux (both of which definitely run at native resolution).
macOS Mojave 10.14.6
MacBook Pro 15” 2019
Radeon Pro 555X
Intel UHD 630
Right, you have discrete graphics. I do not which is why I am stuck with the MacOS driver bug for Intel display hardware.
I’ve seen zero reports of anyone successfully running 5120x1440 using Intel graphics.
Reports I read stated that while you can select it with SwitchResX it was scaled.
I never tried installing it myself because I’m not a fan of modifying the system on a Mac, especially one I don’t own.
From my poking around I think the horizontal resolution is the problem. The system scans possible resolutions to see what works. Apple just never expected a single display that wide.
There’s some reports that newer MacBooks with discrete graphics on Catalina can indeed run this resolution. It used to not work regardless of hardware, now apparently discrete graphics MacBooks can run it. Maybe because they updated the drivers/system for their new super fancy monitors.