Some messages undecryptable in private conversation.

### Description

Matthew and I are participating in a conversation in which he is unable to see my message decrypted, but I'm quite able to see his.

### Steps to reproduce

I'm using both RiotX and the Riot-Web to participate in this conversation, same user account on both. Messages from the web client are undecryptable to Matthew, RiotX works fine. I can see everything decrypted on both clients.

Describe how what happens differs from what you expected.

Logs being sent: yes/no

### Version information

Riot-web current (Firefox Nightly, as of today) and RiotX current as of today.

For the web app:

- **Browser**: Firefox Nightly, current as of today.
- **OS**: Windows 10 current.
- **URL**: chat.mozilla.org