Navmesh is excluding terrain areas it shouldn't

I have a very simple terrain with a ever so slightly bumpy surface.
For some reason patches with almost no slope are being excluded from the navmesh that have extremely subtle slopes (~1-4 degrees), whereas much higher slopes work normally.

I'm using Terrain Collider, Unity 2018.3.4f1 with these settings.

There are no other objects at all in that patch that is being excluded

Step size has nothing to do with it (I've tried stupidly big/small values)

UPDATE IN NEXT POST

 

Nevermind, the navmesh was fine, it was just clipping the terrain.
This was hard to see on a 4K monitor.

Which begs the question, is there a way to make sure navmesh previews are not clipped or that the verts/edges are much more visable?