How to reproduce:

1. Open attached project
2. Open scene test.unity
3. Open the Navigation window
- Make sure the Agent Radius is about 0.05 and that the ThursdayTruck gameobject is marked as Navigation Static
4. Build the Navmesh
- Note that the truckMAT1_002 and truckMAT1_005 gameobjects do not have any Navmesh generated on them, while the identical objects right next to them do have a Navmesh generated on them

- Reproduced in Version 5.4.0b24

- By Design: Use "Min Region Area" Under Advanced bake setting to tweak the minimum size of navmesh - e.g. set it to 0 to keep small areas.
- See also: https://docs.unity3d.com/Manual/nav-AdvancedSettings.html
- This page also explains why the areas does exist for two of the wheels in the report (the area straddles a tile boundary - the bright regular grid of lines on the navmesh debug geometry)