How to reproduce:
1. Open the user's attached project "navmesh_issue.zip"
2. Open Assets/Scenes/modifier_issue.unity
3. In the Hierarchy window select "NavMeshSurface"
Expected result: Navmesh modifiers does not influence Navmesh areas above them
Actual result: Navmesh modifiers influence Navmesh areas above them on Asset edges
Reproducible with: 1.0.0-exp.1 (2020.3.13f1, 2021.1.13f1, 2021.2.0b1)
Could not test with: 2018.4.28f1 (NavMeshComponents package unavailable), 2019.4.35f1 (compilation errors)
Notes: Navmesh agent height does not influence the height influence of the modifier