keycloak container fails with capitalized email characters

Reproduction:

1. Go to our app's user registration page running latest hardened keycloak (see image details below).
2. Enter email with cap characters and attempt to register a new account.
3. User receives this error:

We are sorry... Unexpected error when handling authentication request to identity provider.

Docker logs shows this line as the source of the traceback:

17:30:27,458 WARN [org.keycloak.services] (default task-29) KC-SERVICES0013: Failed authentication: ValidationException [errors={readOnlyUsernameMessage=[Error [error=ValidationError{validatorId='up-username-mutation', inputHint='username', message='readOnlyUsernameMessage', messageParameters=[]}]]}]

Registration using lower case email works.

Setup: This image: docker pull registry1.dso.mil/ironbank/opensource/keycloak/keycloak:15.0.0

Build Date: 2021-08-23T05:00:45.437Z