Is setting preview weight the correct way to use splat maps?

I’m having a right old time of it trying to use a splat map from World Machine on a UE4 landscape. I don’t want to do any painting - just use the splat map to drive texture placement.

Following the tutorials by setting up a landscape layer blend node, setting the layers to height blend and hooking up the splat map channels to the height input didn’t seem to work at first because the landscape was black. What I just figured this morning is that if I set the ‘preview weights’ to 0.33 in the layer blend node (I have three layers in my map) then it seems to work, with my textures appearing on the landscape.

I’m wondering if this is the correct way of using this node though, because when I try using any of the landscape layer visualisation modes in the viewport (debug etc.) the landscape is always black. It’s like UE doesn’t know there are any layers there at all.

If I’m doing something fundamentally wrong I can’t tell what it is from the landscape technical guide or any tutorials I can find. Any advice would be really appreciated.