When I first used Apple USB-C AV adapter (MJ1K2AM/A) on my Mac mini 2018, there were several problems. But I found these are just some setup issues.

HDMI: I use my Mac mini with 2 Ultrafine 4K (by TB3) and 1 Dell P2415Q (by HDMI), all 4K @ 60Hz. I want to test the HDMI port of this adapter works. When I insert this adapter on a 3rd TB3 on my Mac mini, and connect its HDMI port to P2415Q, there is no video output.

The solution is, just unplug one of the Ultrafine 4K monitor (from TB3). It seems the HDMI port on this adpater only works when there is at most one other monitor plugged in (by TB3). And yes, when use this adapter, the monitor can only run 4K @ 30Hz.

USB: the USB port on this adapter works perfectly when I insert it to the TB3 port of Mac mini. However, when I insert it to the USB-C ports on the back of Ultrafine 4K monitor, the USB port doesn’t work.

The solution is simple: just unplug the TB3 cable of the Ultrafine monitor from Mac mini, then insert the adapter to the monitor, and finally plug the TB3 cable to Mac. I guess there is some data re-negotiation happens.