Aha, fantastic, thank you

I have another issue very similar, occasionally, diffuse textures will save to PNG without SRGB, meaning they are very washed out. They look fine inside windows, but inside both Marmoset Toolbag and Unreal 4 they are washed out - the SRGB flag does nothing for it in Unreal, both on and off, the image stays the same. The odd thing is, this is only occasionally. Sometimes I can make a small change in SD to the file, then resave out and boom - SRGB has gone.
Turns out the issue is using 16-bit. Saving as 8-bit solved the issue and used to correct gamma correction.