Thank you Paul for your quick reply.
I just found this forum-topic for May 2020:
Sportstech F37 / gradient Zwift App
Hello, since the software update of Zwift 3 days ago, the gradient is no longer automatically adjusted when I run. What do I have to change in the app to make it work again?
It sounds to me, that this function exists generally, but stopped with the mentioned update.
It exists for biking and works perfect, so why shouldn’t it work for treadmills? 
Best regards,
Stephan