Wikimedia Bugzilla is closed!

If a username with underscores is passed to Special:GlobalUsers, the corresponding user won't be shown in the list.  However, the form on the results page will show the username correctly normalized, and clicking the submit button again will produce the expected output.

The immediate issue could be fixed by adding an extra normalization step, e.g. using Title::makeTitleSafe() like Special:ListUsers does.  However, the actual underlying problem is that the way the GlobalUsersPager class extends UsersPager is messed up and needlessly (and sometimes incorrectly) reimplements much of the parent class's functionality.  Simply removing some of the redundant code and instead reusing the existing features of the parent class would fix this bug as well as various other inconsistencies (such as bug 19131, I suspect).