iPhone timer will not create a sound
I ran into a situation where the timer would not create sound. All it did was vibrate for about 1/10 of a second. Note this similar community question: what to do if my timer doesn't work? - Apple Community was never explained. I have the answer.
I'm running iOS 15.7.3 on an iPhone 6s. It also happens on an iPhone SE that I have, running the same OS. (This is the latest one for this phone.) It also happens on my iPad running iOS 15.6.1.
I discovered that it only happened when the time was set to exactly 5 minutes. Not 5 minutes and 1 second. Not 4 minutes and 59 seconds. The setting for "Auto-Lock" inside "Display & Brightness" in the settings menu is also set to 5 minutes. This is the problem. The timer won't ring when you set the timer to the Auto-Lock time!
So to reproduce this yourself, set the Auto-Lock time to 1 minute and then try to make the timer ring after one minute. After starting the timer do not touch the phone. You'll see that it will not create a sound.
[Re-Titled by Moderator]
iPad Pro, iPadOS 15