Well, there's a bug in answerhub. If I type in a specific string my comment gets deleted. Gonna type it out again, this time ultra short version.

 1. Cull distance makes no difference
 2. LOD set to Foliage, no difference.
 3. Not mesh specific.

Exact Steps and Symptoms:

 1. Add foliage type to the foliage painter tool (as opposed to adding a static mesh directly and painting with that without saving it as a foliage type with the little save icon on the thumbnail). This bug only happens with foliage types to my knowledge.
 2. Paint the added foliage (in my case the vines). Eventually the bug starts happening.
 3. Replace the mesh in the foliage type asset, the bug is still there.
 4. Create a NEW foliage type, right click on the thumbnail of the buggy foliage type in the foliage painter, hit Replace With > MyNewFoliageType. The new foliage type retains the bug.
 5. Do the same replace, but instead of a new foliage type, use one that already exists in the scene, effectively merging the foliage types. The existing foliage will now start exhibiting the bug, as if it picked it up from the buggy one.

My best guess is that there's some invalid transform that causes the cluster to get culled.

Let me know if you need any more clarification.