The answer is here.
Basically, someone messed up and there's a difference between your /var/opt/gitlab/mattermost/data/teams, and/var/opt/gitlab/mattermost/datateams directories. They are the same thing, but someone somewhere named it incorrectly.
I just had to move my profile pictures I wanted to keep, into /var/opt/gitlab/mattermost/data/teams/xuy78sdfuhjhsdf/
as the new users/ directory.
Be sure you chown -R mattermost:mattermost your users directory if you've moved it as sudo.
Hope this helps someone