This issue occurs when the section and material slot order do not match
In this sample project:
According to a report from the licensee, the order of Sections seems incorrect.
According to my investigation, it seems that copying is always done in Section order.
It seems necessary to get and set the Material ID to execute correctly.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-81838 in the post.