Just try to roll back to 18.3.4 and report this to AMD maybe they would add it back in next version
My best guess is MS broke the Presentation Mode (Windows Key + P) 1803, like completely, this will be controled by a app called DisplaySwitch.exe located in your Win/system32 Folder. You will also find dpiscaling Digimonutl, and all things monitor related there. The TrustedInstaller is probably the owner , or a random default user, sometimes in win10 caller unknownUser123.xxx.xxx..xx is generated ....
Anyways head down there and take a look, ill bet not even a folder icon is associated to it. Reassign that and anything else you can think of to Full Permissions Trusted Applications and System, and assign ownership to SYSTEM.
If you run into trouble with that or not to puter savy let me know, i can walk you through a easy way.
Ok cracked it. Going into custom resolutions set timing standard to "CVT- Reduced Blanking" instead of "manual" and it allowed the creation of 1400x1050 as a custom resolution I can now use