I've got Android 6 (Marshmallow) with Messenger+ ver 6.0.6 and am dealing with all VZW text targets. The problem is...
1) I have an ongoing conv with Joe alone (conv #1).
2) I have another ongoing conv with Jane (conv #2).
3) I start a 3rd conv to both Joe and Jane (intending a 3-way group MMS) and send them a message (conv #3).
4) If Joe replies to conv #3, the group msg, my Messenger+ tries to receive it into the Joe Only conv #1, not into group conv #3.
5) Same thing for Jane's reply to #3, is received into conv #2.
6) Worse... their replies that end up in the wrong conv get received as just 'download' buttons. If I tap 'download' it says 'downloading' forever and nothing comes in.
Note: This all worked fine up to recently, maybe 10 days ago. The three conversations all stayed separate and worked as expected.