Why do World Machine tiled landscapes load under the grid line?

I’ve been searching for weeks for some way to raise this island above the green line, where the base grid is. I’ve managed to move the tiles on the X and Y axes with the World Composition panel, but never the Z axis. Why not?

Resetting the Location and Scale to default in the Details panel did nothing whatsoever.

I tried raising the terrain in World Machine with a Clamp and reexporting. It still loaded at the same level, a full kilometer below the surface, in UE.

I tried stretching out the areas that are supposed to be underwater to raise up the parts that are to be above in World Machine, but it still loaded the same scale in UE.

I can spawn and play on the peaks above the green line in the attached screenshot, but trying below the green line makes the camera spawn above it and freeze.

Attempting to drag the blue arrow up in the side viewport gives me a red crossed-out cursor. Dragging anyway locks up UE for a minute or so, with no result.

I’m in UE 4.18. The heightmap tiles are 8x8, 2017x2017 resolution each.

What am I missing?

I just tried reexporting the island at 2x2 tiles, 2017 resolution each. It loaded into UE at about 4x4 kilometers in size, while I constrict the 8x8 tiled structure to about 9x9 kilometers because I don’t really a bigger space than that.

It raised everything up so that exactly the same portion of the peaks could be above the grid line as with the larger tiles.

This seems pretty silly.

Am I supposed to manually brighten each of the heightmap tiles to raise them in UE?

So, lightening the heightmaps in Photoshop did, in fact, work. However, I’ve now compressed them into half of the available total grayscale range. That is a ridiculous loss of detail and data.

Process and result screenshots attached.

Why does UE force half of the grayscale range to be useless for heightmaps in tiled landscapes?