The issue: A modular, to the epic skeleton rigged helmet is not properly aligned when using the larger player mesh.
Description: While most modular helm items fit to a modular setup head mesh, this goblin helm is too low and not properly centered per default. The first thing I tried was to setup a dedicated head socket, assigning the helm, which usually works. However, even though the head mesh has no collision, with the dedicated head mesh socket for the helm, the result was always slightly off, no matter if a SetLocation, or AttachComponent to socket was used or a combination. In the latter the Master Pose appears to not work with attached components at all. Manually tweaking the socket location on the skeleton only worked for the Z value. The result looked like the smesh was colliding with the character mesh, because it is too low, and subsequently this results always in a randomly wrong end-position.
The fix: Because modular skeletal meshes are usually rigged to the root bone, a root bone socket was setup with the helm assigned. Now custom adjustments to the socket work in all directions, and it gives the desired expected result.
Summary: Common socket setup voids standard workflow with root rigged skeletal meshes, which divert from the character mesh, hence non-conformity. By using a root socket the usual workflow applies and yields the desired results. The expected desired behavior would be that all skeletal socket alignments work in the same fashion.