I got it! The heightmaps (created with both Gimp and Blender) were 8-bit png. Just tried with a 16-bit png image that didn’t create that odd cubed output.
There may also be some things that could be adjusted in the way you make your heightmap depending on the particular third party software you’re using (ie. World Machine), but I’ve not used any of those really to give you any advice on where to look. If you are using a third party software for your landscape heightmaps perhaps a user here in the community would know off hand what you’d need to adjust to get a smoother result.
Thank you for your clear explanation, now I got it more deeply.
You’re true, I’d like to use Blender to create the heightmap and surely someone in the community more experienced could point the right way to proceed out to me in order to improve the results.
Bye!