This is a stretch, I know, as I don't have a lot of information to explain why this is happening. But here is what I have:
On 3 different P4's (2 2Gb and 1 4Gb) running a current version of Buster (as of May 2020), running headless 24x7, sometimes they will just act like they've received a poweroff command, at random. Only one machine is running at a time (production environment), but the multiple machines are listed because I've swapped them out to try and determine if the hardware was the problem. I've tried with multiple official Raspberry Pi power supplies on each machine, and never get the lightening bolt on the screen (headless, but HDMI output goes to a television broadcasting system for kiosk use). The (mostly) consistent part is that they are all running the same software, Buster w/x-windows and Chromium as the primary application. I've tried different SD cards (Samsung and Sandisk), and still have the same problem (same image, mostly). One iteration even meant re-installing everything from nada, and it still had the same powering down issues.
On the Pi's the red light is on with no activity being shown on the green led. SSH can no longer connect, and the mouse/keyboard will not get me any response. It's just a black screen with no video signal at all. I already know it is NOT doing screen blanking, as I've turned that off and sometimes it goes a week between these incidents. The only way I can get the Pi back is by unplugging it and plugging it back in.
I've monitored the memory usage. Chromium is a hog, but is really only using about 900M, so I've got 3 gig of free memory. The main differences between this setup and another kiosk using a Raspberry Pi 4 that I can see are: The working (as in doesn't power down) system only has one tab open in Chromium, versus 3 tabs in the failing system. The working system connects to Google slides and not a locally hosted web page. The failing one has 2 tabs using localhost content, with only one tab at a time currently playing a video (also local). The 3rd tab on the failing machine is used to connect to a UV4L camera on another machine (Raspberry again), and when it is connected the other 2 tabs are paused. A bash script is running in the background to switch tabs based on the presence of the UV4L machine on the network, or time of day for which set of videos are playing on one of the other tabs. I've also tried having it reboot itself in the early morning hours, but that doesn't make any difference as it will power itself down less than 8 hours later, sometimes. It's still a kiosk, just a little smarter than the one that's working consistently.
Again, mutliple machines have the same weird problem, using different SD cards (and different brands), along with different power supplies. The consistency is the basic image of Buster, with minor differences as I've fine-tuned what videos are playing, and the tab order in Chromium.
Has anyone else run into an issue of the Pi 4 shutting itself down unexpectedly with what seem to be solid power supplies? Oh, and I cannot get it to happen in my home office, only in the production environment. And the other Pi that is working is plugged into the same outlet at work, and connected to the same QAM converter, and uses the same power supply (and I've even swapped between the 2 machines). I've had it on for 3 weeks solid at home without any failures, but it dies within a few days of power-cycling at work. Don't you love consistency? And let's not even get into the "it only seems to stop when the pastor wants to use the camera, not the rest of the staff" part.