Landscape Shaders Unknown by Landscape Thumbnail: Add to AllowedShaderTypes or ExcludedShaderTypes

When packaging Elemental Demo and Vehicle Game for Linux from Windows, multiple Landscape Shader warnings are encountered complaining that the shader is unknown by the landscape thumbnail material and requests that the shaders be added to the AllowedShaderTypes or ExcludeShaderTypes.

There are a couple warnings that precede the Landscape Shader Warnings:

LogMaterial: Warning: D:\Unreal Projects\ElementalDemo418\Content\Elemental\Effects\FX_Post\M_ColorCorrect_Mountains.uasset: Failed to compile Material Instance with Base M_ColorCorrect for platform SF_VULKAN_ES31, Default Material will be used in game.
LogMaterial: Warning: D:\Unreal Projects\ElementalDemo418\Content\Elemental\Destruction\CrackingIce\M_CrushedIceTop_Dark.uasset: Failed to compile Material Instance with Base M_CrushedIceDark for platform SF_VULKAN_ES31, Default Material will be used in game.

This issue is a regression as the ability to remote build from Windows to Linux in the Binary build is a new feature for 4.18. This issue was encountered in conjunction with [Link Removed].

1. Open Elemental Demo or Vehicle Game in UE4 on Windows
2. Package the project for Linux in the Development build configuration
3. Watch the output log for warnings of the following form:

   LogLandscape: Warning: Shader TMobileBasePassPSFMobileMovableDirectionalLightCSMWithLightmapPolicyINT32_MAXHDRLinear64Skylight unknown by landscape thumbnail material, please add to either AllowedShaderTypes or ExcludedShaderTypes
   

Expected Result
The package process completes successfully without errors or warnings

Actual Result
Multiple Landscape Shader warnings appear asking for the shader to be added to AllowedShaderTypes or ExcludedShaderTypes

Is Unreal's CDN overloaded or failing?