Repro steps:
1. Open QA attached project "repro_1078153"
2. Open scene "0"
3. Enter Play Mode

Expected: Cylinder moves over the inner NavMesh Modifier Volume which has a different Default Area than the outside of the Platform
Actual: Cylinder stays in place

Reproducible with: 2017.2.3p4, 2017.4.11f1, 2018.2.8f1, 2018.3.0b1

Notes:
- Swap the Default area's of GameObject "Platform" and "NavMesh0", rebake the mesh and see that it gets colored correctly in the Scene View

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Note from Unity devs: Be aware that the area type with the higher index prevails when sources overlap (be it geometry or modifiers). This detail is mentioned in the manual page for "Navigation Areas and Costs".