I'm also having this problem - tested working on a MacBook Air 2020 (Intel) w/ 11.4 and it works fine. Tried on a MacBook Pro M1 w/ 11.5.1 and only see 1080p. When I select 'scaled' I see no other options.
Same displays work fine on Windows, it appears to be related to latest MacOS update.
Setup: Dell D6000, 2x 4K Dell displays connected via HDMI and DisplayPort
Update: On an Early 2015 MacBook Pro 13" running 11.15.1, I have no problems with the same Dell D6000 dock and same monitors. Only difference is it's connected via USB-A and not USB-C like my M1 MacBook Pro.
system-profile.jpg
4k monitor only one option.jpg