iPhone 15 Timer Function Turns Off After First Use

I have an iPhone 15. I have recently had to use the timer function 4x daily, 7x week. At least a couple of times a day the timer sounds off after the first use. When I reset it, it turns itself off. I manually start it for the second use and then reset for the final use. No problems for the last reset.

Any ideas? Restarting my phone doesn’t make any difference.

[Re-Titled by Moderator]