Same problem with instant messaging programs like Signal.
Messages are not delivered when app is in backround.
With april update it worked only in wifi not in mobile.
With may update it worked almost well.
With june update it didn't work at all.
Phone seem to close app in backround after a while.
I put the application in the blacklist backround to see if it change something.
Maybe I'm silly but think it's better when blacklist...but it's non sense...
I'll test it more during the day.