Board Thread:Support Requests - Designing Your Wikia/@comment-26917042-20150829171453/@comment-25428589-20150829174154

It's because you've set MediaWiki:Welcome-user to your welcome message. That's not what that page is for - it decides who leaves the welcome message. That page should contain either the username of the admin who will welcome users, "@latest" for the latest admin to edit (or staff if there are no recently active admins), "@sysop" for the latest admin to edit (without the staff thing of the previous one), or "@disabled" to turn off the welcome tool.

See this page for more info.