Spigot raises spurious `PlayerTeleportEvent`s when players travel through portals. This is particularly a problem for the WorldBorder plugin.

With the server instrumented with a plugin containing the following code:

    @EventHandler()
    public void onPlayerTeleport(PlayerTeleportEvent event) {
        plugin.getLogger().info("onPlayerTeleport(): " + event.getCause() +
                                " from " + formatBlockLoc(event.getFrom()) +
                                " to " + formatBlockLoc(event.getTo()));
    }

and similar logging for `PlayerPortalEvent`, the logged output is:

onPlayerPortal(): NETHER_PORTAL from (world_nether,296,68,321) to (world,2370,68,2568)
onPlayerTeleport(): NETHER_PORTAL from (world_nether,296,68,321) to (world,2370,68,2568)
onPlayerTeleport(): UNKNOWN from (world_nether,2370,68,2568) to (world_nether,2332,65,2466) 
onPlayerTeleport(): UNKNOWN from (world,2332,65,2466) to (world,2332,65,2466)

The second to last `PlayerTeleportEvent` has `TeleportCause.UNKNOWN`, overworld X, Y and Z but says that those coordinates apply in the nether, which is the wrong world.

This is possibly related to #1807, but not the same.