The actual mode has to be 3840×2560, not 1920×1280.
(The "looks like 1920×1280" HiDPI setting is then automatically added by macOS.)
After adding the actual mode, assuming it shows as "Active", switch to the "Available resolutions" tab and select the 1920×1280 HiDPI mode whose refresh rate matches the one of the 3840×2560 mode you just created.
Last edited: