From what I recall if you’re working in a PBR workflow in SD the maps that get output are already in linear space, even if your intermediate images are in sRGB.
So when you bring them in to Unreal leave sRGB off.
However, if you have a convert to linear node in substance you’re performing the sRGB to linear conversion first, then again when the map is output, so turning on sRGB in Unreal just undoes the second gamma correction and gets the final texture back to linear space.