So much for my educated guess....
Well, if I hadn't looked at the webfig internals recently... I might have agreed.
See
viewtopic.php?t=79771&hilit=webfig#p1007098
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.