I also was struggling hard with this issue. I had this problem while connecting a Samsung Viewfinity S8 4K display directly via USB-C. I was using BetterDisplay (what I really can recommend to you all having problems).
The highest HiDPI Resolution I could get was 1080P. When using HDMI as connection, I could select all resolutions as HiDPI (I wanted to set my external Monitor to 2560 × 1440 HiDPI).
The solution was to go into BetterDisplay Settings for my Monitor and set a Configuration Overwrite for the Native Panel Pixel Resolution (which was shown as 720P). After entering 3840 x 2160 Pixels and rebooting, I could select all HiDPI resolutions also with my USB-C Connection.
Perhaps this helps somebody having the same issue.
I found a screenshot of the Settings Panel here: https://screenshots.macupdate.com/JPG/63642/63642_1656264874_scr_uc1.jpg
Regards, Bjoern
b_plutka
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