Dunes solution fixed the problem, so thanks again!
But now I run into another problem that shouldn't be occurring...
I have only a few surface layers, and I wanted to add one.
I had a surface layer for rocks, and decided that is would be good to give it a child layer.
- But I can't -
I mean, I can, but then the hightfield disappears.
When I create a surface layer node, and plug it into the 'child layers' input of the rocks surfaceshader, the height fields disappear from the preview window.
When I unplug it, they reappear.
And it's not just the preview window. I rendered it with the node attached and the render had just flat surface.
I think this counts as a bug.
I tried adding a child in the Shaders tab menu, but that gave the same result.
Placing the new surfacelayer below the rocks shader fixed the problem, but it still shouldn't happen.
Below are some images.
First is the node view, with the problem shaders circled.
Next is what the scene looked like,
and below that is what the bug's result looks like.
I also included the tgd file