Can you check also to see if the original texture file that were imported as 16 bit and not 8 bit? There is a known issue with exporting textures from the engine that were imported as 16 bit.
Never gotten an email about this (it seems rather picky/random) on what answerhub emails me about.
The problem only occurs with imported 16 bit .PNG files btw.
This issue was reported and amazingly already fixed but it looks to be slated (barring any quality assurance problems that may arise) for release with 4.8