From GHI: https://github.com/keycloak/keycloak/issues/10461

Fix for https://issues.redhat.com/browse/KEYCLOAK-18518 solved session leak for standalone-ha configuration, however it seems that the same problem occurs for standalone configuration and it is not fixed. It is even mentioned in original issue that issue affects both standalone and standalone-ha modes however it seems that fix was applied only for standalone-ha.

More details in mentioned GHI.