My main question here is, are you importing heightmap or are you creating one within the editor?
At this point I might need you to zip up your project and share it with me so I can see what is occurring on my end. I prefer you upload it to a shareable google drive link so I can download and have a look.
Also, in case you had not seen the documentation yet, we do have some solid documentation on Landscape Materials and the nuances of creating landscapes. Take a look at the links below for more information.
Let me know if you have further questions or need additional assistance.
Thank you,