The 3 flavors of the Monitor (Counter, Gauge, String) fail the same.
Refer to the 3 JCK tests:
api/javax_management/monitor/MonitorComplexAttribute.html#MonitorComplexAttribute05
api/javax_management/monitor/MonitorComplexAttribute.html#MonitorComplexAttribute20
api/javax_management/monitor/MonitorComplexAttribute.html#MonitorComplexAttribute35
(attached the 3 jtr files)

The test monitor an attribute and the getter for that attribute on the observed object throws a runtime Exception.
We expect a notification of type jmx.monitor.error.runtime and since Mustang b53 we receive it of type jmx.monitor.error.type.