David Brennan

Got a screenshot of the landscape in UE with the ‘Paint’ tool selected, you should have your ‘water’, ‘debris’, … layers in there.

Thanks Damien, appreciate it.

So I searched through the Unreal Engine C++ and the holes are controlled / created by a Visibility layer that's added to the landscape, this is a separate non-material layer that's only added if needed.

After that I took a look through the Houdini plugin C++ code and it looks like Houdini will translate the Heightfield data into the height layer for the landscape and then any other layers (rock, flow, …) will be converted into material layers.
It looks like there's nothing currently there that will create the visibility layer.

If anyone can confirm my findings, that'd be great.
I'll keep digging and if I find anything else I'll update this topic for anyone that finds it in the future.

Cheers,
Dave