Nav mesh bounds volume only works correctly in some orientations

I am trying to construct a navigation mesh for a world in UE4, and I’ve placed a navmeshbounds volume in the map. But strangely, the navmesh works okay for some ‘orientations’ of this volume, but for other orientations, the nav mesh is being computed for areas outside the volume as well. See below, where the long black rectangle indicates the navmeshbounds volume:

Correctly computed:

Wrong (out of bounds nav mesh computation):

Why is this happening?