The function UClothingAssetFactory::ImportToLodInternal that set the FClothPhysicalMeshData structure is not call again when we re-import so the vertex colors are not updated.
Import attach fbx "SK_Sphere_VertColor.fbx" with create physic asset true
Select the unique section and create a cloth asset from it
Activate the cloth painting
Select the asset
Copy vertex color in the mask call "none" to paint cloth from vertexcolor (use Red channel)
save all asset (material, physic, skletalmesh and skeleton)
Re-import with new file the asset use the attach fbx file "SK_Sphere_VertColor_2.fbx"
open the cloth painting and copy again the vertex color in the mask (Use Red channel)
the mask do not take the new vertex color
The mask has cloth in the bottom instead of at the top
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-81442 in the post.