Windows ClearType Text Tuner informs me that, "Your C49RG9x monitor is not set to its native resolution" and offers to change it to its "native resolution of 3840x1080. This was also an issue with Windows 10.
I suspect that other subsystems in windows also get this wrong as whenever I load a game (like Halo MCC) I have to manually adjust the resolution from 3840x1080 to its actual resolution of 5120x1440.
I have an RTX 3070 TI card, and this issue has also been verified to exist on other desktops using an RTX 2070 and also intel UHD onboard/integrated GPU (using the same monitor) .
My m1 macbook does not have this issue, nor does ubuntu 18.04 or ubuntu 20.04, so it's fair to say it is NOT a monitor issue.