Since 18.4.1, custom resolutions do not appear within games. This includes even 1080p. Only some resolutions and downscale options native to the monitor are available.
Note that I am referring to the unofficial BootCamp editions of the driver of course but the bug seems to be on AMD's side as it was working ok in 18.3.4 and it appears, at least in the installation code, that the way to add them into the registry hasn't changed.
Running the latest Windows 10 April update too.