I had 6 nodes in a cluster with 2 NMs stopped. Then I put a host into YARN's yarn.resourcemanager.nodes.exclude-path. After running yarn rmadmin -refreshNodes, RM's JMX correctly showed decommissioned node count:
After restarting RM, the counts were shown as below in JMX.
Notice that the lost and decommissioned NM counts are both 0.