IOS stopwatch halts - but continues on lock screen

I'm on IOS 26.3.1, an Iphone 13. I have the following issue with the stopwatch (also before the last upgrade). Stopwatch starts, the iphone sleeps. Then after some time (say 15 minutes) the stopwatch is running as it should on the lock screen. When unlocking the phone (either by tapping on the stopwatch on the lock screen or by normal unlocking) the stopwatch app has stopped. When going back to the lock screen it still is running. When starting the stopwatch in the app, it shows up in the lockscreen as another lap.