Because they use different profiles for resolution negotiation with the monitor, the hdmi must have found 4k support in the HDMI 1.2 or 2.0 profile and switched while the dp routine found the lowest common denominator with the monitor, like 640x480 and used that.

The gfx bios isn’t that bright.

First thing I would try is turning off CSM in your UEFI/BIOS settings. If that doesn’t work or is already disabled, I would try this out