I want understand such a situation. Timer in the standard watch application used to behave unstable. The situation was such that when I started the timer for the first time (after last turning on the phone), the timer sound wasnt not working at the end of time (only silent notification on the top). Another situation was that the sound was sounding for one second and then suddenly stopped. It was exactly the next time after the silent case. And after that the sound could work properly. It seems to me that clearing the cache and application data helped to solve the problem. But why did this happen? I can well expect this to happen again in the future. The only option I changed was turning off the vibration in timer settings. My phone is LG Spirit 4G LTE (LG-H440n), Android 6.0, timer sound: "Timer"
Phone wasn't in the silent mode, All volume sliders were up, No task killer
...It happened again. In the same way