When a player enters a nether portal with no existing exit portal, a new portal is created automatically but no PortalCreateEvent is sent.
PortalCreateEvent does have a possible CreateReason.OBC_DESTINATION that is described in the docs as "When a portal is created as a destination for an existing portal when using the custom PortalTravelAgent.". This leads me to believe a PortalCreateEvent should be sent in this case.
A PortalCreateEvent is sent, as expected, when a player lights the portal by hand using flint and steel.

Steps to reproduce:

• Create a plugin that listens for PortalCreateEvents,
  PortalCreateEventListener.java

  class Testplugin extends JavaPlugin implements Listener {
      @EventHandler
      public void onPortalCreate(PortalCreateEvent event) {
          getLogger().info("Portal created!");
      }
      @Override
      public void onEnable() {
          getServer().getPluginManager().registerEvents(this, this);
      }
  

• Start a server with the plugin installed (and a clean, new world).
• Connect to the server and create a portal using obsidian and flint and steel. This triggers a PortalCreateEvent correctly, with CreateReason.FIRE.
• Walk through the portal. This should trigger a new PortalCreateEvent with CreateReason.OBC_DESTINATION as the exit portal in the nether is created automatically, but no event is sent.

I discoved this while trying to find a way to stop portals from automatically forming in specific areas.