The messageCount being negative was an issue with last-value-queues that is being fixed on the next version.
https://issues.jboss.org/browse/HORNETQ-466
with this commit:
https://github.com/clebertsuconic/hornetq/commit/a78836cdef4e28d76064500f57cb8e8a799da9bf
Other than the negative counter, everything works as expected.