The original world is used to create a new ShapeDetectorShape instead of the desired world in EntityPlayer#a(WorldServer). In the patch for EntityPlayer, it is at this line.

Note: To test this, you need a world that is not part of the default set.
If you use the attached plugin, it will create a test world on it's own.

Using the Test Plugin:

• /test_portal_event = Set a meta flag to sent you to the test world