Some hosts are not shown in some Dashboards (All Hosts, and Agents on All Hosts) on the Federation Master server.
The Dynamic Model Service needed to be re-synced.
RESOLUTION: Restart the Federation Master server to re-sync the Dynamic Model Service.
We gathered the output of the attached groovy script. The script output showed that the missing hosts were in the host model (listed in #!(HostModel).hosts#.getTopologyObjects()), were listed in the #!Host#.getTopologyObjects(), but were not in 'All Hosts’ system service (FSMSystemService), which the Hosts page uses. Since the #!Host# query returns the object the Dynamic Model Service should add the hosts to the DMC when it next processes that DMC. Unfortunately there aren't any operations on the Dynamic Model Service to get it to re-sync the associations. The simplest way to trigger a re-sync is restart the server.