I ran into the same problem a few years ago: There is nothing in
GET _nodes/stats telling you if a node has dropped out of the cluster. And Kibana monitoring will only show the node (in gray) for the duration of the selected Time Range (1 hour by default), after that it will be gone from Kibana surveillance too, until it rejoins the cluster.
My solution was to create a simple cluster deployment configuration, which I found useful for other things too (such as full cluster restarts), in which I list all the nodes that should be in a particular cluster. Then a monitoring script uses this configuration when checking
_nodes/stats and reports if a node is missing. This requires a bit of scripting but is a straight forward job for a programmer.