OcclusionRoughnessMetallic SurfacePainter [as Linear] material into Unreal 4


firstly just to say I am also posting in allegorithmics surface Painter forum as this could well be an issue on their side.

But. using a OcclusionRougnessMetallic texture in the fashion as documented here [where the texture is non RGB [i.e linear] and Occlusion = R, Metallic = B and ROughness = G]:

I get the following visual artifacts on my shader as per attached.

IS there something I have missed? from The shader preview and viewport views it seems that this texture [the linear one] is somehow creating these visible artefacts
but I have no idea why…

Any help would be much appreciated.