I have a couple apps I use regularly on my A71 called 3CX and Discord. When I am connected to wifi and have not used one of these apps in a few minutes, they seem to "sleep" on the wifi, where I will receive notifications long after actually receiving the notification in the app.
Maximizing either app to use again, I receive a message that they are "connecting". I've put both these apps into the "never sleep" category and set them both to "allow background activity"
Is there something else to try?