Here's what I mean in pictures. My mesh already has vertex color info ("colorSet(RGBA)"):

So I activate the "Keep Vertex Colors" option (because I want color information to be preserved):

But As soon as I start moving the slider then the additional (black/empty) vertex color set gets created and added to the mesh. So the result is the new (black/empty) set becomes the active one and negates the existing one:

Could you please take a look at this issue? Thanks!