I have a situation where I cut a long box into the navmesh. This works if Is Dual is not ticked but the breaks, if it is enabled. See screenshots for details.
Is Dual off:
Is Dual on:
The bluish rectangle is above the navmesh and the cuts should be parallel to it but you can clearly see that the cut is not rectangular but rather consists of triangles connected by lines.
Using Version 4.2.17.