I'm having this issue with my Mac mini (late 2012) Quad-Core Intel Core i7 2,3 GHz running MacOS Catalina v10.15.7.
I have an Apple Thunderbolt display and HDMI monitor attached to it.
Changing the power saving option solves the issue but I don't want to continue using this "workaround" for my Mac mini.
***Power save settings when issues occured:***
Computer sleep: after 30 mins
Display sleep: 15 mins
Crash/reboot issue occurs after waking up the computer from sleep mode.
***Updated Power save settings - no more issues:***
Computer sleep: ***never***
Display sleep: 15 mins
Crash/reboot issue no longer occurs after waking up the display.
***Conclusion***: the issue is clearly coming from the Mac mini resuming from sleep mode since it no longer happens with the updated settings that doesn't put the Mac mini to sleep mode. I don't think it has anything to do with the OS since some people are seeing this with High Sierra but I'm using Catalina.
Current workaround power save setting is not my preferred one since it keeps the mac awake and is consuming power all the time.
I will probably be doing some more tests in the coming days/weeks.