If the user attempts to utilize a procedural foliage volume over a terrain, and static meshes are below that terrain, say in a cave for instance. The procedural foliage will also spawn there as well.
Have you added a foliage blocking volume around the meshes in question? The procedural foliage volume will spawn meshes on any surface that falls within the parameters of the volume. So if you, for instance, tell the foliage that it should only spawn on grass materials, it will be limited by this. If there are no limitations set, it will assume any surface within the defined area can accept foliage and act accordingly. In this instance, the best thing to do would be to use foliage blocking volumes to prevent the foliage from spawning within the defined area.
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