I have researched and found this is a common issue. It's always the same person. Each time I UNMUTE conversation, the REPLY mutes it. I've played games trying to fool my contact list that it is a different person, but one way or another it aways discovers who it is and mutes the converstation. I have tried many other apps on my Samsung 7 edge and they work fine, however, I like Message+.
I called tech support and they couldn't fix the issue and never heard of it anyway. Just do a google search and you will hear of it!!
How do I unmute the conversation.