The Custom Side Vector facing mode is no different than the Custom facing mode on Niagara ribbons. The linked UDN has more information on a potential cause, as well as a proposed fix. It looks like in FNiagaraRendererRibbons::SetupMeshBatchAndCollectorResourceForView the vertex factory's facing mode is used before is set.
The ribbon is facing the same direction with both facing modes.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-89777 in the post.