Repro steps:
1. Open "PathError.zip" project
2. Load "Test" scene
3. Open the Navigation window and select the Bake tab
4. Bake Navmesh

Navmesh does not connect around two blocks if the Plane object is under the "floor_01" prefab and set to Not Walkable/Jump/Dirt, if you disable Plane object in the hierarchy or set it's Navigation Area to Walkable it connects correctly.

Actual: Navmesh baked on a prefab doesn't connect between cubes if the Plane object under prefab is set to Not Walkable/Jump/Dirt.
Expected: Navmesh baked on a prefab should connect between cubes if the Plane object under prefab is set to Not Walkable/Jump/Dirt.

Reproduced on: 5.4.1p2, 5.5.0b3
Did not reproduce on: 5.3.6p5