That sounds like the default placement for the mesh itself is wrong. Does this happen when you drag & drop the mesh onto the scene?
It should be easy to add an VerticalOffset value to AFoliageTileActor and then do something like ‘HitData.ImpactPoint.Z + VerticalOffset’ here: https://github.com/hallatore/TileActor/blob/master/Source/Landscape_1/Private/FoliageTileActor.cpp#L187
Feel free to make a pull request of it 