Thanks @beckf for the report
We made a change to properly validate the usernames when creating users via the API: https://github.com/mattermost/platform/pull/5581
However, it looks like it's causing issues with LDAP usernames as you reported. We have a ticket to revert this change, which is considered for Mattermost v3.8.1: https://mattermost.atlassian.net/browse/PLT-6334