Active directory user names should not be case sensitive. For example if I add the user "paul" then I would expect to be able to also login as "Paul", "pAul" or any other case combination.
At the moment this won't match any user name and fail to login the user in, or worse match none and apply "authenticated user" permissions. This seems to confuse a lot of users.
If there is a use case where case sensitivity is required then I think there should be a toggle option to enable or disable it.