Hello,
We are also experiencing similar issues where someone is signed in to their Mattermost, but will show up as another user when chatting with someone else. Today we had a user who had Private Channels list that belonged to another user because it knows he was not apart of any of those private channels and his private channels were gone. Doing a refresh resolved issue, but this is not good. We are utilizing Mattermost that is built in to Gitlab using Gitlab login as access to Mattermost. How does mattermost sessions identify back to individual users.