Thanks for getting back to us with that information, Edward. It's appreciated.
The first thing I'd like you to check is that "Stop Playing” has been selected when the timer ends.
Use the timer or stopwatch on iPhone -- Open the Clock application, and check the settings for Timer. Make sure that "When Timer Ends" that "Stop Playing" has been selected.
Restart your iPhone, iPad, or iPod touch -- If the setting is correct, restart your iPhone. Once it's returned to the Home screen, see if the Timer works as expected now.
How to back up your iPhone, iPad, and iPod touch -- If restarting your iPhone didn't resolve the issue, please be sure to have a fresh backup to keep your personal information safe.
Update your iPhone, iPad, or iPod touch -- Since iOS 13.3 is available, please update your iPhone, then see if the Timer works as expected.
Get back to us when you're able.
Cheers!