17 inch iMac resolution 1280x720

I have a 17 inch 2012 (Late) iMac (3.4Ghz) (13,2) with Nvidia GeForce 680MX. A few days ago, I noticed the resolution was not set to native 2560x1440, so I went into System Preferences to change it back and saw that the only available resolution is 1280x720. I have recently upgraded to Mojave, but I have had no resolution issues since the upgrade until this happened.

I have tried using a variety of EDID tools to dump the display settings as well as try to update them but to no avail. The system only allows 1280x720 scaled. It is the only entry available, other than 800x600 if I press option while selecting the Scaled radio button. There is no native resolution option.

I have gone so far as to install Windows using Bootcamp, to see what Windows detects and Windows sees only 1280x720 as well.

I have reset NVRAM and SMC etc. dozens of times while troubleshooting this.

I have also tried SwitchResX to no avail, it seems to go native but all looks pixelated.

If anyone has any clues, or a sample EDID file from a working Late 2012 iMac with my config, or SwitchResX settings, or simply confirming some hardware problem or known issue, it would be very helpful.

I have searched and search Google and see some references to this issue with various outcomes, but nothing concrete.

Thanks,

Mark.