I have copy pasted user login when assigning user to a role. It went with some whitespace around it. I was able to add user like this just fine. Entry was resolved to full user name as expected but permissions granted to the user are not applied by Jenkins.
Seems like user check should be consistent with permission application to the user. I would probably vote for trim of whitespace from entered user name. Or it should not be bale to resolve the user. Or at least display warning there is whitespace in login and results can be weird.