This may be a limitation at the moment, but landscapes will block any AO generated from Distance Fields for anything that is below the landscapes surface. This can cause issues for anyone using a cave system with their meshes and using landscape in conjunction, similar to the user reporting this issue.
Attached .umap from the images below. (created with 4.9 CL-2593205)
1. Open UE4 and enable Generate Distance Field meshes in project settings
2. Change light mobility to Movable
3. Add a skylight with mobility set to movable
4. Add a landscape
5. Place a mesh below the landscape
6. Viewport > Show > Visualize > DF AO to see results
Results: Landscape doesn't allow DFAO for any mesh that is below the surface
Expected: DFAO should allow ambient occlusion below the surface of the landscape, similar to using a large static mesh above the area you want to have ambient occlusion.
Head over to the existing Questions & Answers thread and let us know what's up.