NavMesh not building around z height

I added box colliders to a few static mesh ground planes that came with a house. Now my problem is, that I have to move those away from z == 0 to get the navmesh to work. Placed at z == 0, none of the 5 planes generate a navmesh. Moving all of them up to 6, 4 of them start building one, the last one still doesn’t until put to 7. On the opposite, when I move all of them down to -114, the same with the 4 and the fifth comes in at -115. That’s a total difference of 121 units within which the navmesh does not build. It large enough though (as seen in the screenshot)
Whats keeping the generation from just happening everywhere?
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Ok, it seems it had to do with the size of the the NavMeshBoundsVolume. Shrinking it made the mesh work. I’m wondering why that is though.