Well, if I hadn’t looked at the webfig internals recently… I might have agreed. 
See http://forum.mikrotik.com/t/webfig-auto-login-show-only-some-data/72342/1
The OP’s issue seems like a some race condition where the password manager is setting the username field on page load, but some of Mikrotik’s JavaScript populates the username field in background – e.g. AFTER the password manager already set the field…
As feature request, it would be nice to set the “default user” in RouterOS config, especially since I’d say its a “best practice” to NOT use “admin” as a login name, but it being default only encourages it . Even if possible with branding kit, that’s awful complex & not 100% it work since I haven’t tried doing that.