Mac Mini 2018 locks up, then powers off

My Mac Mini worked fine for 10 months and then developed this problem where it shuts down by itself. For all of that 10 months I had the Mac Mini booting off the internal SSD, with an external Thunderbolt 3 drive enclosure with 4 HDs (OWC Thunder 3 Quad Mini) running without RAID. One of the drives is an 8TB Time Machine backup, the other 3 drives have projects and whatnot. iTunes and iPhotos point to those external drives, but the system folders and /user are all in the SSD. I also had an HDMI connected 4K HD monitor running in full resolution as my only monitor.

About 6 days ago the Mac Mini developed a problem where it would stop outputting video. Initially I thought it was just shutting down all at once. I didn't let that happen too many times before I disconnected my external HD array. The computer wouldn't drive its display even during boot? At that point doing Pram reset might have been interesting. Instead I booted into Rescue and attempted to re-install the OS. This seemed to work. I booted into the not-so-fresh Mohave install. It still had my two log-ins and my desktop files. But when I went back to my usual stuff (with 4K monitor and my external HD), after a short time (under an hour), the system would freeze and then shut off the monitor. The last thing I did was attempt to update to Catalina OS during which I was away from the computer. When I came back the monitor would not light up. So I unplugged it.

I stopped messing with the Mac Mini and took just the computer to the Apple store. I took it into the local store on Sunday 10/27/2019. The tech who saw me hooked up their own HD monitor, i think it was HDMI, not sure. And their own keyboard and mouse.

I told the tech that it wouldn't start up (to the point of displaying) and that the last thing I'd try to do was to get the regular boot (into Mohave) to update to Catalina OS - I did not see that install complete. I detailed the symptoms which included not booting, locking up mouse followed by sudden computer-off, lower-half of the screen turning to fuzz for 1/2 second or so during boot, Apple icon and progress bar turning blue for short periods during boot.

The Apple tech demonstrated the fuzz, but now the computer booted into Catalina just fine. It also shut-down suddenly while I was messing with it. The tech used Control-opt-P-R to clear the PR memory and did some magic tech stuff.

The Apple tech did some research on his tablet and told me that the problem was that the computer wasn't shutting down, it's actually just turning off the monitor and this is a known problem with the Mac Mini and 4K displays. With remote SSH access I can turn of the computer from the iPhone (Telnet app). Also plugging and unplugging the monitor will temporarily restore operation.

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What I figured is that if this is all true, I can live with it because the problem has always existed and yet I got 10 months operation out of the machine before the first time it happened.

So... i went home.

All of this was wrong. The computer is freezing and shutting down by itself. It is also not supporting any monitor reliably.

When I turn it on it says it was shutdown badly and offers to re-open apps. It asks to Report. I say yes. It shows a screen that says:

Your computer was restarted because of a problem

Problem Details and System Configuration says

"BAD MAGIC! (iflag set in iBoot panic header), no panic log available.

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There are several other weird things going on.

The computer completely freezes up, no mouse movement, no screen updates, and the magic trackpad 2 stops clicking. Then several seconds later it turns off. You can tell from the power light.

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Debugging this...

There are articles here in the communities about 4K monitors giving the 2018Mac Mini fits.

However.. I don't seem to have exactly that problem.

If I run this with an HD monitor on a USB-C to DVI adapter, and with no external drive, it seems to be stable for days. If I plug in the external drive, the computer shuts down within an hour.

Today I left it running with no external drive with just the HDMI 4K monitor hooked up, in 4K mode.

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As of this morning I'm testing the assumption that the problem I'm having is the Thunderbolt controller in the computer. or maybe the actual files on the external drives. Maybe Spotlight crawling through those files screws up somehow? Dunno.

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If the computer does not hang up today running no external drive but with the HDMI 4K monitor (I'm away from the Mac Mini for 10 hours or so), I'm going to take all of my drives out of the Thunderbolt 3 enclosure and then plug it in. Then next step might be to add some empty drives and mess around with that.

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Has anybody else seen this?