This is fixed now. The avatar shows the person who started the conversation (as the message is the first message).
On a technical note, getConversationForUser now includes the conversation starter information. This is consistent with getConversationsForUser (plural) so I don't think it should cause problems. However, if you've extended getConversationForUser to work with xf_conversation_starter, this could lead to an SQL error after upgrading. (This is needed to fix this, but also to make the queries more consistent.)