This affects Craftbukkit and Spigot 1.14, not 1.13.2.

Load up a clean server with the provided plugin. Create a nether portal and step through. This should normally say the cause is NETHER_PORTAL, but it currently says UNKNOWN. Now create an end portal (eg. /setblock ~ ~ ~ end_portal). This should normally say END_PORTAL. Now when returning from the End through an end portal, it should also say END_PORTAL, however currently nothing displays as the event doesn't fire.

Test code used in provided plugin :[^TestPlugin-1.0.jar]

public class TestPlugin extends JavaPlugin implements Listener {
   public void onEnable() {
      getServer().getPluginManager().registerEvents(this, this);
   }
   
   @EventHandler
   public void onPlayerPortal(PlayerPortalEvent event) {
      Bukkit.broadcastMessage("Portal event caused by: " + event.getCause().name());
   }
}

(regarding #8 of the bug report instructions: spent an hour on this trying to be exhaustive, verifying the bug in the code, setting up workspaces and building)