When a nether portal is broken, an observer will not send out a signal. This can be reproduced by simply having observers facing the portal and a redstone lamp.

Expected: The redstone lamp should flicker on when the portal is lit and when it is broken.

Observed: The lamp is powered only when the portal is lit, but not when it is broken. Also, if you are in creative, only breaking the portal tile right in front of the observer will trigger a signal, but any other portal tiles will not.

The image below is the setup I used for testing this. it should be easy to reproduce.