If the skeletons are the same it shouldn’t be happening… Even if they have different proportions (but bone names, order and default rotations are the same) it does work because I’ve tested on Epic’s Paragon characters that have big proportions differences, but use the same bone hierarchy as the mannequin. Sights align normally, because what the code does is to place the bone in front of the camera, overriding the original bone position as commanded by the anim sequence being played.
So I suspect the problem might be in the animation sequence that was generated for that skeleton when it was retargeted, because this is a problem I did see occur already. So you only see it when you use that skeleton, but because you only use that anim sequence when you use that skeleton…
Anyway, whether the issue is in the skeleton or in the animation, it’s something under the hood in the engine that I wasn’t able to discover. Might be related to additive layers of translations/rotations to make the different skeletons compatible with the different animations, but it’s a bit beyond my reach, unfortunately…