Active app being sent to background - why?

I was just reminded that I've fought this battle before:

https://forums.developer.apple.com/thread/82830

Maybe I'll trying upping the timer tolerance again.

My disappearing app problem still happens with the timer tolerance set to 180, my timer code below.

When my app enters "NightMode", I disable the Idle timer to keep the app active and in the foreground all night, and I turn on the proximity monitor to allow the screen to go black when the phone is placed face down.

        UIApplication.shared.isIdleTimerDisabled = true
        UIDevice.current.isProximityMonitoringEnabled = true

A user's log shows that the .isIdleTimerDisabled = true was correct at 1:36AM and again at 1:46AM. The log then goes blank, meaning the app was "dead". My app is heavily logged for debugging and won't do hardly anything without making a log entry. After 1:46 there was nothing about any of these events, which should all generate log entries:

applicationWillResignActive
applicationDidEnterBackground
applicationWillTerminate

The app came back to life and logging in the background at 4:11AM this morning. It reported that the .isIdleTimerDisabled = false at that time. How did it get reset? What the heck happened to my app after 1:46AM?

Starting my timer for NightMode:

    weak var timer: Timer?
    func startTimer() {
        timer?.invalidate()   // just in case you had existing `Timer`, `invalidate` it before we lose our reference to it
        if #available(iOS 10.0, *) {
            timer = Timer.scheduledTimer(withTimeInterval: 600.0, repeats: true) { [weak self] _ in
                self?.timer?.tolerance = 180  // Upped from 80 on 5/27/20
                let dateFormatter = DateFormatter()  // format the date for output
                dateFormatter.dateStyle = DateFormatter.Style.medium
                dateFormatter.timeStyle = DateFormatter.Style.long
                let convertedDate = dateFormatter.string(from: Date())
                self?.logSB.warning("The timer triggered at \(convertedDate)")
            }
        } else {
            // Fallback on earlier versions
        }
    }

How can I diagnose this? Unexpected demotion to the background continues to plague me. It happens only rarely and may only affect some users.

My app is the active one, but suddenly is sent to the background. I've tried to use logging to sort it out but I see no evidence of low power, no memory warning, nothing except the .applicationWillResignActive event.

In the most recent event, background operation commenced and app refreshes continued normally. But I need the app to stay active when that's what the user expects!