Redis sentinel doesn't auto-discover other sentinels
In configuring the sentinels, I configured sentinel once and then copied that configuration file onto all the others.
Before copying the config I must have started up sentinel and it re-wrote the config file with an id like:
sentinel myid c766a0df56e72eda48cea984
When I copied the file to the other sentinels it caused each of them to have the same id, which in turn caused them to ignore any messages from the auto-discovery with that same id. Removing the myid line from each config file and restarting each sentinel process fixed the issue.