An update this thead:
I a running internet recovery now (Shift + Option + Command + R) to reinstall macOS Mojave.
The problem here is that even with disconnect / reconnect the HDMI port never works in this very early stage of macOS in recovery mode. Putting a Apple Thunderbolt to HDMI adapter between the Mac mini 2018 and the Apple HDMI to DVI adapters works.
This is so annoying, getting a Mac mini 2018 which was original working with a original Apple HDMI to DVI adapter with was and is perfect working with Mac mini 2010 and 2011 here in the office with this HDMI to DVI adapters to our 16:10 office monitors.
Back to the original story. This was working for about 2 month, the Mac mini 2018 was in the lab for deployment test.
Then after two macOS updates / patches the HDMI in port broke.
The only way to get the Mac mini 2018 back to a working HDMI adapter was to change the logic board.
This was done twice. The replacement logic boards came with macOS Mojave and after two updates the HDMI port reproducible broke.
This device is now 19 month old, we where only able to work this this device for testing for 2 month.
Five weeks ago i remembered the stories the Steve Jobs sometimes responded to customers emails so i tried to email to Tim Cook. The outcome was interesting. After four days i got a call and an email from a Apple Executive Liaison manager who coordinates this issue now. And three days later i was nearly daily working with a Apple support engineer working on this problem. Doing tests, collecting diagnostic files. Making photos and videos of the issue.
Testing with different monitors, testing with HDMI connected projectors to compare.
Taking diagnostic files again and again. But no success.
We know that the issue starts with a update of the adapter firmware.
We do not know if its possible to install an older firmware version.
So for now it looks like the only way to get a Mac mini 2018 with working HDMI port with Apple HDMI ti DVI adapter
is to replace the logic board or replace the Mac mini 2018 with a fresh one and never update.
The support engineer tries to find out if it would possible to block especially the graphic adapter firmware and lets run the other macOS updates.
No i reinstalled macOS Mojave from the internet recovery and the issue is the same.
The interesting point is, that directly after the macOS Mojave installation the graphics adapter firmware was 2.35 and after my tests with filevault and voiceover to unlock my Mac mini 2018 it is now 2.0a.
Now i am updating to the latest macOS Mojave version which is 10.14.6, lets see what happened.
So we have reproduces the problem. Even if i use a Apple Thunderbolt to HDMI connected with a Apple HDMI to DI adapter, which did show a screen with Filevault disabled, there is now no screen visible.
Its impossible to unlock a HDMI broken Mac mini 2018 which has Filevault enabled without using voice over to hear where and when you need to type in username and password to boot up the mac.
What do next?
The Mac mini 2018 we have is now 20 month old. Still not working properly.
It was physically repaired twice by replacing the logic board.
It has gone through many hours of testing in the lab and with Apple support team.
It has Apple Care for 36 month, to there are 16 month left to get a working device....
And there are many users which reports the same or similar issues here in the forum.
Using a second adapter is possibly ok for many users.
But the need to enable voice over and let anybody in the office hear when you start your Mac mini 2018
and type in your username and password is not an option.
This is #macmini2018hdmigate. That needs to be fixed.
We need to replace our older Mac mini devices. But we will not throw out all of our office monitors because of an broken HDMI port of the Mac mini 2018.