I've noticed this issue for some time, but I'm creating this ticket now after I ran into it and when the "cutoff" resulted in pointing to a user account that, by coincidence, existed.
When navigating to certain special pages or even some pages, usernames that contain a '?' within the string will be interpreted as part of the HTML as a parameter and hence gets cut off and removed from the input into the appropriate field. The issue looks to involve the MediaWiki software and its interpretation of URL input when you navigate to pages that involve usernames that contain a '?' somewhere, and may possibly affect usernames with other certain characters within them that are interpreted as part of the HTML link.
For example: If you navigate to the following pages, you'll see what I'm talking about:
In each case, the URL itself isn't altered at all (at least in my browser; I'm using Firefox), but the MediaWiki software cuts off the username at the '?' thinking incorrectly that this portion of the URL is a parameter being passed to the page, resulting in only the "Wantsex" portion of the username being automatically filled into the appropriate filter or username input text field. Don't get me wrong, I'm sure that this "cutoff" is needed for certain links and pages to navigate users properly, but in these cases it's causing issues at times.
Creating a phab ticket to document the issue and have it looked into.