I just purchased the 2018 Mac Mini, and also a LG 32UD99-W monitor that does 4k (3840x2160) at 60 Hz.
When connecting HDMI cables from the Mac Mini HDMI out port to the monitor HDMI in port, the picture has a very "codec compressed" look. By compressed, I don't mean that the picture is squished. Instead, it looks very similar to what a desktop would look like over a VNC/Screen sharing application. Everything is somewhat blurry and "bleedy", especially when looking at red colors.
I don't have this issue when using the monitor's USB-C or DisplayPort. I have tried with two HDMI cables, first one of which I'm not sure its version, and then a HDMI 2.1 cable with these specifications:
Ultra HD 7680x4320 (8K) at 60 Hz or 3840x2160 (4K) at 120 Hz. Dynamic HDR, HDCP 2.2, Deep Color and 3D.
Both cables have the same issue. I have read that HDMI should not be different in picture quality compared to USB-C/Thunderbolt/DisplayPort but this is clearly not the case – I'm just wondering if all HDMI transfers are "compressed", or if it’s the monitor alternatively the Mac Mini in tandem that doesn't do a good job at HDMI?