I have exactly the same problem that started a few days ago. The Play store says it updated itself April 18th, supposedly to fix bugs and add new features, but it clearly created new bugs. Whenever I open the Message+ app, I get 10 or 20 loud beeps that used to come one at a time whenever a message arrived in the background. Now they seemed to be saved up, and perhaps duplicated, when the app is opened.
I never noticed what version of message+ was installed before but it is now 6.8.2. My phone is Mot Z2 Play, Android veresion 8.0.0. All App permissions are turned on for Message+. Restarting the phone doesn't solve the problem-- when the phone finishing restarting, it starts beeping like crazy if there are any new messages that have arrived in the background.