Mask handling in AE can be kinda squirrelly. Left click the mask (click on the actual mask line) on the segment between the 2 new vertexes you created, and make sure its selected; it should “highlight” and you should see the curve handles etc.
Then try using the application menu (while the segment is still selected):
Layer-> Mask-> Closed (de-select)
Ethan Anderson
I CAN Learn Education Systems
http://www.icanlearn.com