New external user propagation to mattermost

Summary

When I add a new user and click the "external" button, it takes a very long time before the user is available in mattermost.

Steps to reproduce

1. Add a new external+regular user, click external user, do not allow new groups
2. Add him to a project
3. In Mattermost, create a new team. Add users to the team.

The new user is missing from the list of available users.

Eventually they do show up, but I don't know if this was only after an upgrade of gitlab.

Our mattermost is the one bundled with gitlab and we use gitlab for login to mattermost.

What is the current bug behavior?

The user isn't there.

What is the expected correct behavior?

The new user should be there :)