This works - thanks a lot!
It was working for me in K9 5.6 and I thought the update changed it. But it was me 
Now I only need a solution for
https://forum.k9mail.app/t/notification-led-no-longer-works/2014/2?u=stphn
and then all notification issues should be gone for me. But this might be something which I have to wait for in K9 6.0…