in UMaterial::GetAllReferencedExpressions, the mobile branch ignores every CustomOutputExpressions including RuntimeVirtualTextureOutput.
1、Enable Virtual Texture Support on Mobile
2、Create a ordinary landscape RVT map, and do not use VirtualTextureFeatureSwitch in the RVT Material（layer samples are only connected to RuntimeVirtualTextureOutput instead of MaterialProperties）
3、Change preview level from SM5 to es3.1 and you will notice that the es3.1 preview is not sampling any layer's weight at all
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-115033 in the post.