@John
What do you see when you hit the following NameNode JMX call from Ambari Server host?
# curl -s "http://$ACTIVE_NAMENODE:50070/jmx?qry=Hadoop:service=NameNode,name=FSNamesystem" | grep 'MissingBlocks'
# curl -s "http://$STANDBY_NAMENODE:50070/jmx?qry=Hadoop:service=NameNode,name=FSNamesystem" | grep 'MissingBlocks'
What is the Ambari Version are you using?
Can you try opening ambari UI in Incognito Mode (Private Mode) in the Web Browser just to see if the browser cache is not playing any role there?
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