Landscape spline self shadowing not working?

Hello there,

I have a question regarding landscape splines and depth field GI. Currently I am experimenting with hedgerow meshes that are meant to be used with landscape splines, they have a bunch of masked materials and placing them in the level as static mesh makes them look halfway decent with depth field GI and shadows (left side of the image)

Placing them as landscape spline makes the self-shadowing almost disappear (right side of the image), they’re rather bland compared to the static mesh version. Is that a bug? Feature? Something I missed?

Best regards