This is a regression tested in //UE4/Release-4.27 CL#18319896
Update Mesh Section incorrectly sets vertex colors for a Procedural Mesh. The closer a colors R, G, or B value is to 1.0 or 0.0 the more similar the values are between the updated mesh section and the non-updated section.
The square is one solid color.
Half of the square is darker than the other half. The closer the R value is to 1.0 or 0.0 the more alike the 2 halves are.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-157225 in the post.