That's probably because it isn't a proper term/diagnosis.A syndrome by definition is a collection of symptoms, whereas having an unusually 'flat back' would be a clinical sign or observation.
It's generally thought that a flatter spine is better for being static and a more curved spine better for movement.
Does the client actually have any symptoms or problems as a result?
My initial thought is that a flatter spine is probably more suited to lying ( supine or prone) on a treatment table.
Andrew.