I believe that for me it has something to do with the width of the saddle/twist combined with set back stirrup bars.
Same for me. I don't have a twinge on my Dartmoor with Heathers hipsaver on it but without it I'm in agony.
Strange that you had hip pain with the Fhoenix. I'm really stiff but had no pain at all in that saddle. I tried GP version.