Create or open any material in engine
Add TransformVector node to the material graph
Set Source to Tangent Space and Destination to World Space in TransforVector params. Grab the HLSL and put it into a diff tool.
Switch Source to World and Destination to Tangent. Copy the HLSL to the diff tool and view the differences.
The only difference shown is that Parameters.TangentToWorld is cast as "MaterialFloat3x3", which does not really make sense, and visually there's no difference in the material either. (See diff and example material attached)
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-134296 in the post.