When the interface implemented by the MBean class has no MXBean semantic but that interface inherits MXBean semantic from his father, then MBean is MXBean.
The RI rejects with with a NotCompliantMBeanException.
The specification may be clarified; it says:

[...]
Otherwise, it may be an MXBean. The set of interfaces implemented by the object is examined for interfaces that:
[...]

The "set of interfaces" is the whole set, not first level interfaces only.