JMX (open MBean) supports only a limited set of basic data types, and unfortunately, Instant is not one of them. Looks like internally, the JMX serializer enforces this restriction using a hardcoded serialization filter, which blocks unsupported types like Instant.

Proper way to pass Instant is to wrap it in a CompositeData. Interestingly, Instant did seem to work when returned directly as an operator return value. I'm not entirely sure why that is.