A zero sized "LUN" may show up if you don't present a data LUN with a Host LUN ID 0 (zero). In this case the Management LUN (formerly LUN "Z") shows up as LUN 0.
Anyway, you said that you presented LUNs from two storage pools to the same hosts. How did you configure the "Host LUN IDs" for the different LUNs? Are these unique across both storage groups, or are some of the LUNs presented using the same Host LUN ID?
Note: I'm not familiar with the VMAX, but I assume it's similar to a VNX.
André