This is a sRGB color space issue.
Potential solution: Use regular texture import instead of FImageUtils::ImportFileAsTexture2D in ThumbnailGenerator::GenerateThumbnailFromFile
Load attached image, Leather.png, as a thumbnail in the VariantManager.
See in other attached image that the resulting thumbnail is overexposed.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-217539 in the post.