Volume (or non-collidable) PathModifiers parts do not affect Terrain. For example, putting a non-collidable part with a PathModifier does not change the navmesh on the terrain at all.
A video showcasing the issue.
On the left: a non-collidable part with a PathModifier that doesn’t affect the navmesh on the terrain. In the middle: a collidable PathModifier on the terrain.
On the right: a non-collidable PathModifier that does affect the navmesh.
PathfindingTerrainBug.rbxl (61.7 KB)
Expected behavior
I expect that the PathModifier modifies the nav mesh. This behavior is not documented on the PathModifier documentation, so I assume this is a bug: PathfindingModifier | Documentation - Roblox Creator Hub