Usermanager not case sensitive?

I think we found a bad issue - or is it by design?
For occ i am evaluating if the user exists. The problem is that the check is positive. No matter if E.g. Admin, admin or aDmin is entered.

This leads to another issue later, that during initialization of the userview a skeleton is created for a new syntax - but this later. First the usercheck.

Any idea what could be wrong?

if ($this->userManager->userExists($user)) {