This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
When attempting to import a texture that has a non-opaque alpha content and compressing using the "masks (no sRGB)" texture compression setting, pixels that are in the alpha channel are being weighted as being less important.
The pixel values contained within the RGB and alpha channels should be weighted with the same importancy.
The pixel values in the alpha channel are being weighted as much less important than the ones in the RGB channel creating artifacts in this area.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-23235 in the post.