Sounds like a bug in the latest AMD Driver. Best bet is to make a AMD REPORT ISSUE from here: AMD Issue Reporting Form . This way AMD Support will become aware of the bug and hopefully make a fix. Or you can open a Support EMAIL Ticket from here : Email Form . Most likely they will respond by saying to use the last known driver that works correctly. The first link you won't get a response but will let AMD Support know there is a problem. The email you will probably get a response.
Meanwhile I would keep using driver 18.3.2 until AMD issues a fix in a future driver. There is another thread with the same or similar problem.
If you really need to have the latest driver installed for some program to work, then as a work around, you might want to download and use Custom Resolution Utility to restore the missing resolution on your computer. It is a tiny program but quite useful. If it doesn't work then you will be stuck with 18.3.2 driver until a future update fixes the problem.
Edit: What does Windows Settings show under DISPLAY as for your monitor's Resolution range with the latest AMD Driver installed? If Radeon Settings is not showing 1366 x 768 resolution see if it is showing under Windows Settings - Display. If it is, click on the correct resolution and see if the monitor changes to it. If it becomes black. Wait a few seconds and it should go back to the original resolution if it doesn't work correctly at 1366 x 768.
FYI, I reported using a 1366x768 as a bug.
In my case it was pretty simple. I can use a 1366x768 HDMI display just fine using default Microsoft driver on a Ryzen 3 2200G. Using any sort of AMD driver and it will longer even detect the display! Drove me nuts because I was doing a clean install of Windows 10 on a newly built computer last night.