If you're red-teaming a Matrix server and you own up the server, you can decrypt all the messages of all the people using that Matrix server, and you can do it trivially, just using the basic protocol messages. Users might see extra group members in their groups and they might not, depending in part on whether they're running the most current version of the software.

You can decrypt all the messages. I sometimes feel like I must have read a different paper than everybody else.