It might be because it seems that even after you run gitlab-ctl reconfigure, the URI is not updated automatically, so you have to manually re-authorize Mattermost using these instructions: https://docs.gitlab.com/omnibus/gitlab-mattermost/README.html#authorise-gitlab-mattermost
You can find more info about the set up here: https://mattermost.atlassian.net/browse/PLT-4078?focusedCommentId=35670&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-35670