I have used this monitor (HP Z22n G2) with my previous mac mini (2012) and it worked just fine via both HDMI and DisplayPort.

However, after updating my mac to 2018 model I'm no longer able to control my display.

Version 1.5.0 didn't detect display at all, version 1.5.2 detects and shows on-screen HUD, but never actually changes brightness or contrast.

So I assume that this is an issue with Mac Mini 2018 itself. Already tested on other mini 2018 devices and got the same result (so not an issue with this device instance but with model).

Furthermore, I've tested NativeBrightness and have had the same issue there.
Maybe this issues is related to DDC/CI library or DDC/CI control is absent in this model.