Dis you see the documentation here: https://docs.mendix.com/appstore/modules/saml/ ?
In Mendix version 7, all user names passing through the SAML module are converted to lower-case before login and creation. We strongly advise you to convert all existing and new user names to lower-case as well.
Or solve it to create an extra attribute in the account entity SSOLoginName. In that field you could specify how exactly the account name should be and retrieve the account based on that field.