which system function does stop my sports app after 15 min. time?

[AC Question]

I am walking often 1 hour or more. Since years I use a sports application which measures the distance, time, speed, calories, etc. Since beginning of February I use a Samsung Galaxy Folder 2 handy with Android 6.0.1. Since then the system stops the application after 15 min. time. First I thout it was an accu app which stops but after a test, when I stopped the accu app before I started to walk, the sports app stopped again after 15 min. So I guess I system function stopps the app after 15 min. to save power. Has some one an idea who stopps the app and how I can modify this 15 min. parameter?