I have two Mac Minis that I run as headless servers, both started exhibiting this behavior after upgrading to 10.15.6. I never ran into this before, so I thought it was a problem with this latest version of the OS. But now after reading this thread and others, I'm seeing the problem has existed quite a bit longer. I'm not sure why it just started happening for me.
As I said, I was using these as headless servers so there is no monitor or keyboard attached. I purchased a couple HDMI display emulators, to trick the Minis into thinking there is a monitor attached, thinking that might help alleviate the situation, but it hasn't made any difference.
On one computer (Mac Mini Late 2012) the crash and reboot happens multiple times a day, usually once overnight and 1 or 2 more times throughout the day. On the other one (Mac Mini 2018) the crashes happen less frequently, usually once every day or two.
I've read that one possible solution is to display Power Nap. I did that on both, but the 2012 mini still crashed today after making that change. So I don't think that is the solution. I've also read others post about issues with external monitors connected to a MacBook Pro, but that obviously doesn't apply in my case.
Hoping against hope that someone might have a solution, but moreso, I just wanted to add my voice to the chorus. Hopefully someone at Apple will take notice, and we'll get a fix sooner than later.