I have the same issue.
Mac mini 2018; 3.2GHz i7; 16GB RAM; 1TB SSD; Mojave 10.14.5. Purchased June 2019.
Issue: Immediately after boot, upon reaching the login screen there's a 50%+ chance the display disconnects, requiring me to reconnect the entire cable. The interesting part is I have zero issues if using dual-display setup. The issue only occurs when using a single display.
Further, I get "snow/static" (as described in the
other thread) if using a much older Samsung display, but otherwise I get a simple disconnect if using a newer Samsung display.
I've tested every "trick" and configuration possible. 3 different displays, new/old display, Direct-HDMI, USB-C-to-HDMI, old/new HDMI cables, reset NVRAM, reset SMC, SingleUser Mode, Verbose Mode, reset display settings, change input detection, change AV/PC mode, adjust display resolution, confirm FileVault disabled, DetectDisplays button, clean OS reinstall.
The farthest I've gotten is 4 "good" boots in a row without an issue. Otherwise I get a "bad" boot seemingly every other boot. The only times it doesn't happen are if using a single display, when booting into Safe Mode, and when booting from an external HD.
I can see in the kernel log that the system is having some issue with the HDMI "hot plug detection." It almost seems like a timing issue. Perhaps using dual-display changes the conditions allowing it to seemingly work properly. Perhaps the issue is specific to certain types/brands of displays.
Although my dual-display setup seems perfect so far, it still has me very concerned that perhaps this is a permanent defect in my brand-new Mac mini, and has me considering if I need to save myself the grief and just return it. This Mac is meant to replace my 3rd MBP, which is the 3rd machine to have some fundamental defect (2 GPU defects and 1 battery defect.) Wondering if this is now the 4th defective unit. /rant
A user on the Apple forums indicated that he confirmed this is fixed in the Catalina beta, and also a version or two prior to Mojave 10.14.5. So that gives me some hope, but I'll believe it when I see it.
Question: Can anyone confirm that they are successfully using a single-display HDMI connection on a 2018 Mac mini running Mojave 10.14.5?