I will throw in a few ideas, based only on my own recent experience, along with some theories based on zero medical knowledge.
The past month or so I have been upping my workouts on my 4 month old hip, and the more intense activity level seems to be firing up a sore area in my 10 year old hip right where you are describing. That area has been a little weak ever since surgery, but it does respond to strengthening exercises. The optimist in me says that the discomfort is just because the rehab is working that 10 year old resurfaced hip harder than normal. The pessimistic side wonders if there might be some fluid accumulation in the glute, even though my ion numbers are <1. Dr Gross has commented that almost all replaced hips will have some fluid and wear accumulation at some point. Did the doctor who looked over the MRI with you comment whether the white areas were fluid? For myself, I am starting a program using the roller and massage again to hopefully break up any fluid collection, if there is any, and also stimulate more blood supply to the area.
Let us know what you figure out.