You can pin the Landscape HLOD actors in the outliner to open up their materials to see if there are any shader compilation errors. The default Landscape HLOD material is a Virtual Texture material (this can be overridden in your DefaultEngine.ini, but it’s not exposed in the UI), so my first instinct would be to make sure you have Virtual Textures enabled, but in my own testing the landscape HLODs look fine whether or not Virtual Textures are enabled or disabled when building them, or even disabling them after building HLODs, so possible red herring there.
All of the Landscape HLOD settings are set in the Landscape settings, so changing anything in the settings for the HLOD Layer you’ve assigned to the landscape won’t make much difference.