See https://github.com/keycloak/keycloak/issues/12523 which was fixed for Keycloak 21.

On MariaDB / MySQL Upon DELETE USER api is used to delete a user, it looks like following SQL query is executed to delete offline session details from OFFLINE_CLIENT_SESSION table.

This issue is caused by MySQL / MariaDB way of treating subselects, and the Keycloak / RHSSO fix is a workaround specifically for this case.