I just tested the Find Members option and it is working fine, I cannot tell why you will be experiencing an issue with it, except that you could be doing something wrong.
Try using this -
https://bitcointalk.org/index.php?action=mlist to find out if both usernames exist or also just try copying and pasting the username without trying any other thing and changing your profile.
So do I need to rest assured the person 'Username Hey' will not be able to message? or I should keep in my mind the possibility of 'Username not_Hey' to be able to message me ?
If it is on your list, they should not be able to message you.
- Jay -