Basically this line of code tells UE4 to treat the first thing they came across in hierarchy as root bone, regardless of whether it’s actually the case.
If you’re new to the engine I don’t think you should make any PRs.
The real fix would be for the importer to check if the first thing in hierarchy is really a root bone. And/or check if first thing in hierarchy is “Armature”.
There is a lot of “fix” for this problem. For me, this one do the trick.
“As things stand right now, the importer for fbx is effectively patched (a nice word for hacked) to allow as exception for Blender made fbx files if the name of the root node is ‘Armature’.”