The timer will not turn off
The timer will not turn off
[Re-Titled by Moderator]
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What do you mean? Does it snooze instead of stopping? You have to either click the Stop button from the Lock screen, OR tap the banner and go into the Clock app > Timer to stop it. If you just swipe a banner away it snoozes it.
If it's not stopping at all, and just continually playing, try restarting the phone.