For me it just helped to use Power of 2.2. And maybe even this is just needed for previewing purposes.
I know you tried it… but it’s just a proof that in my case it works well 
Unreal version: 4.16.1
I’m trying to boost performance of out game. One of steps is to create atlases.
The oryginal material had two textures. One for Base Color, and the second for Roughness. Both compress as Default.
I decide to pack Roughness as Alpha channel in atlas. This is where problem start. In texture preview old texture and new (alpha) are nearly identical (compression probably), but in material alpha is more then 10x brighter!!
On screenshot below I used multiply by 1 node for preview window.
The texture of intrest is the one in the middle of bottom line on the altas.
Here are the textures to download https://we.tl/tTIQQU6jq5
What is going on. Is it a bug, or a way how editor treat alphas in material editor?!
Normally, I would agree, and it do work in OTHER cases. This is not the first time I use this technique… 