Being on 1 mg for three months is a good amount of time. It suggests that PMR was in remission. Perhaps your cortisol level wasn't enough to regulate inflammation. What happened when you needed to go back to 7 mg of prednisone?

My endocrinologist explained that my cortisol level needed to rebound before I would be able to taper completely off prednisone. She wouldn't predict how long it would take for that to happen. She only said that I would need to stay on a low dose of prednisone for "as long as it takes."

Hydroxychloroquine seems to control PMR for many people. You really need something to replace Prednisone while waiting for your adrenals to recover. I don't know how people manage to get off Prednisone without some type of "steroid sparing" medication.
https://connect.mayoclinic.org/discussion/any-tips-for-tapering-off-prednisone/