org.jboss.annotation.ejb.ClusteredImpl's partition property has a default value of null. Effect is, if you deploy a clustered bean without @Clustered, but instead just use <clustered>true</clustered> in jboss.xml, it doesn't work. You also have to include a <cluster-config> element to specify the partition name, even if you just want the default partition.

Solution is to have the ClusteredImpl.partition default value be ${jboss.partition.name:DefaultPartition} – the same as @Clustered.partition.