Thanks for the reply. I doubt that it has something to do with Android Killing Kontalk because it started suddenly when i first posted here. I never had a problem problem before. I use the Google Play Version for a few Months now and never had a problem. Before that i was Happy with the F-Droid Version. Maybe i will go back to that.
Just today i had another Message send to me 4:15 PM and i "received" it 4:28 PM after opening Kontalk because i was expecting the Message. No Notification whatsoever. All other Messages that day had no problems. This really puzzles me.
My Device is a Moto G2 xt1072 with a Monthly LineageOS Build. I build it every Month as soon as the Android Security Patches for the Month are included. But as i said, it started of of the Blue (the ROM i use currently had no issue till i posted here. A new rom will be installed in a few days i guess. Depends on google and LineageOS).
And yes webratte, I speak Dschörmähn Kontalk is not Battery Optimized. I always do that since the F-Droid Version to make sure it works ok.