We just had an update from Lithium. Auto-suggested search got turned on by default. I turned it off. Let me know if there are any further issues. Also, keep in mind that if it does suggest something different, it also provides a link to search the exact string you input.
Also, make sure that you don’t have auto suggest turned on for yourself. Below the search field, there should be a link that says, “turn on suggestions.” If it’s not there then you need to click on the empty search field to turn it off.
I don’t know if you noticed this before, but I’ve never been able to search a username without switching the search filter to Users. So, I’m wondering if that’s part of the issue. I will post this to Lithium and see what they say.