iPhone 6 plus @3x resource on navbar with incorrect alignment

I have an issue with iPhone 6 plus compatibility. For an UIBarButtonItem's image on the navigationbar I applied two versions (@2x, @3x) in imageAssets catalog. On the storyboard I set image inset to the barButton to allow 5px right margin. This is being achieved well on all devices, other than iPhone 6 plus. I have also tried to modify the alignment on the resource itself, on imageAssest, though no matter what value entered, no changes were seen.

Question is, why wouldn't the margin apply on iPhone 6 plus as well? Where could the problem be?