UIApplication.shared.isIdleTimerDisabled not working all night?

I have more data on this problem now. I put my iPhone face down last night at 11:51PM. My log file shows that the proximity detector was active and I saw the screen dim as I laid it down.

My timer is set to run repeatedly every 610 seconds after the proximity detector is turned on, which it did successfully ten times until 1:33AM. (That's 142 minutes and the math works out.)

At 1:36AM, my app responed to a background fetch, meaning it was in the background at that time and the timer was no longer running. Background fetches continued every few minutes until 1:51AM and didn't resume until 7:14AM. They then continued every few minutes.

What happened at 1:33AM in the morning to stop my timer and put the app into the background? Is there a better way to ensure all-night running?

Here's my timer code. I didn't set the tolerance previously because I couldn't find syntax for doing that without errors until now. Maybe that wold help?

    weak var timer: Timer?
    func startTimer() {
        timer?.tolerance = 60  // This does not throw an error but I haven't tested it.  
        timer = Timer.scheduledTimer(withTimeInterval: 610.0, repeats: true) { [weak self] _ in
            self?.logSB.warning("The timer triggered at \(Date())")  // I use SwiftyBeaver logging
            self?.didEnterForeground()  // This triggers UI changes and other actions
        }
    }
  
    func stopTimer() {
        timer?.invalidate()
    }

It seems to be more reliable now that I've set the tolerance on my timer, as above. It's run throughout the night several nights in a row.