I watched that stream too! - Here’s a timestamp of what’s happening to some of your bones in First Person. (So this could definitely be related to that voodoo needed to adapt a third-person model to first person)
I did some quick tests and found attaching to ‘Custom Effect Socket’ “Base” to be the most stable. Although hopefully in the near future we wont need to abuse the VFX system to attach meshes. (Scene Graph save us?)