Under some circumstances, Coherence cluster nodes that run within the same logical partition (LPAR) on z/OS on IBM zSeries cannot communicate with each other. (This problem does not occur on the zSeries when running on Linux.)
The root cause is that z/OS may bind the MulticastSocket that Coherence uses to an automatically-assigned port, but Coherence requires the use of a specific port in order for cluster discovery to operate correctly. (Coherence does explicitly initialize the java.net.MulitcastSocket to use the necessary port, but that information appears to be ignored on z/OS when there already is an instance of Coherence running within that same LPAR.)
The solution is to run only one instance of Coherence within a z/OS LPAR; if multiple instances are required, each instance of Coherence should be run in a separate z/OS LPAR.