Hello. My question is about unrealEngine4. I have runtime generation in dynamic and everything (navMeshVol and navModifier) is correctly positioned in your place.
I made this example as a test. Click on play, delay of 10 seconds and by direct reference of the nav mes modifier I change its area class.
During the 10 seconds delay, I run the show navigation command to see the navigation mesh, but it does not change. I have created a new project and I still have the same problem.
En mi caso, lo arreglé desactivando initial building locked in proyect settings/navigation system.