Data node cannot find master node

I don't know, have you? Are you still seeing warnings being logged about not finding the master? Does GET _cluster/health report that both nodes are in the cluster?

By "they don't indicate anything that needs fixing" I didn't mean "nothing needs fixing", just that those specific messages don't indicate anything needs fixing.

@DavidTurner GET _cluster/health reports that I have 2 nodes in the cluster. Yay!

Great news. I suggest you remove logger.org.elasticsearch.discovery: TRACE from the config and restart the node since you won't need this extra logging any more.