Blocky landscape after heightmap import even when using 16bit?

Hello! I try to import a heightmap created in World Machine (simple Radial Grad just to test it), but the result is blocky like that:

I use the Landscape Import Heightmap from RenderTarget function:

RT is not a problem, because it renders e.g. material with Noise node smoothly.
I see in the function tooltip that the “In Import Height from RGChannel” input needs to be checked to avoid precision loss, so maybe that’s it? But when I check it, then the landscape is just flat.

That’s the heightmap, it’s 16bit, 2k, and I exported it as 16bit from the World Machine:

Here is my small test project (sorry for the name):

I’m new to the Landscape workflow, so sorry if that’s something obvious, but I’m out of ideas. I’d be grateful for any help, I’m stuck for now.