I think it’s a macOS issue in terms of the boot screen. It’s a really weird resolution at 3840x1600. It’s not even true 4K at 3840x2160.
Apple doesn’t really care to support it which is sad. We should send them feedback for Mojave.
The Display Port Boot Screen issue is definitely not a macOS issue.. it exclusively affects the early system boot (splash screen, clover and verbose boot messages) and not the MacOS System. Same issues when booting the system with Windows or macOS! I am sure the issue is due to the Display Port 1.2 implementation on the monitor and the Display Port 1.4 implementation on the Vega. And basically it is some incompatibility between monitor's and Vega's firmware..
The 30 Hz HDMI issue indeed seems macOS related, although the need for enabling HDMI compatibility mode on the monitor for a proper monitor wake again rather points to Display Port or Firmware incompatibilities between the Monitor and Vegas..
I never observed any of these issues with my LG 38 inch, when using my former Nvidia GPU and the respective web drivers.
And the issues really relate to
ALL Vegas!
I guess the responsibility definitely is at the side of AMD/ATI and LG or any other 4K or WUHD monitor manufacturer still implementing the old Display Port 1.2 and HDMI 2.0 standards..