It sounds like the MX, in your environment, is maybe not acting as the DHCP server for VLAN 100, but is relaying to a different (centralized / offnet?) DHCP server? In which case you'd maybe be better looking at that DHCP server..? If an MX is acting as DHCP server, the active allocations etc. can be viewed from the Appliance status page, DHCP tab near the top (as @RWelch pointed out) - under All DHCP leases, put 100 in the 'Filter by' window
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