I ran into an issue where converting a heightfield to a polygon mesh using the Heightfield Convert SOP leaves values in the uv.z component. This ends up causing baking errors and artifacts when you try to bake down the heightfield in COPs. You can easily fix it by dropping down a wrangle to set uv.z to 0, but it's pretty unclear why it happens and took me quite a while to figure out! I have added an example file.