The following was observed through the MMS agent.

A replica set lists its members by IP addresses. Unfortunately, 2 hosts had the same hostnames. That did not create any issues from the mongod point of view, but it did confuse the MMS agent, which builds a list of host mappings. The list ended up being a union of the lists from both hosts.

The customer changed the hostname on one of the 2 hosts. The information reported from that host through the last ping in MMS stills shows the old hostname.

If we allow for changing the hostname without restarting mongod (and without reconfiguring the RS) then we have a cached value somewhere of the hostname that is not updated.
If we do not allow changing the hostname without restarting mongod, then it works as expected.