Steps to reproduce:

1. Send a private messages between 2 users (s1 and s2).
2. Create a group conversation with, at least, these users (s1 and s2).
3. Send a message to this group conversation (you'll need the patch in MDL-63303 for sending them).
4. Open the private conversation of these users in the mobile app (you can use https://mobileapp.moodledemo.net/ for that).

Expected behaviour: Only the private messages between s1 and s2 are displayed

Current behaviour: The private conversation between s1 and s2 also displays the messages sent to the group conversations where both users are members of.