Vertex Color are broken in 5.6 on some meshes.
This is mesh vertex color in 5.2 (works as intended):
This is vertex color of the same mesh (no matter if mesh is migrated from 5.2 or imported via new or old importer):
And this is same mesh, but in vertex painting mode. In this mode vertex colors are correct.
But on final materials vertex color is white (or sometimes color is similar to needed but distorted).
This causes visual artifacts like this (sharp horizontal division between gray and green is expected here):
