Earlier we used RTC 4.0.5 and used to fetch contributor without any problems (immaterial of case sensitivity)
Code Snippet:
public IContributor getContributor(String userId, ITeamRepository repositoryHandle) {
        try {
            
            IContributor contributor = repositoryHandle.contributorManager().
                            fetchContributorByUserId(userId, null);
            return contributor;
        } catch(Exception ex) {
            log.error("Exception occurred while getting Contributor ["
                    + userId + "] " + ex);
        }
        return null;
    }

We are planning to migrate to RTC 5.0.1 and during testing we find that contributor retrieval is case sensitive.
API is able to fetch contributor only if userId is provided as seen in Active Directory.
Eg:
My user ID as seen in Active Directory =  SSS1KOR
Result seen on execution of  above function
Input (userId)            Outup (Contributor)
SSS1KOR             Fetches Contributor @......
sss1kor                          null

Note:
Login with web Client is successful immaterial of case sensitivity.