It could be the prednisone, but it could also be the PMR. I had a lot of problems with one shoulder before I was diagnosed with GCA and PMR. The movement of that arm was very restricted and painful. But I've also had problems with muscle strains while tapering prednisone.

Here is response I got in google to the question "Can PMR cause tendon problems":
"Polymyalgia rheumatica (PMR) can cause tendon problems, specifically tenosynovitis, which is inflammation of the tendon sheaths, and may also lead to rotator cuff tendinopathy."

Here is a link to a case report where a rotator cuff cuff rupture was the first symptom of PMR - https://austinpublishinggroup.com/clinical-case-reports/fulltext/ajccr-v3-id1095.php

I was on a high dose of prednisone for the GCA, and my shoulder problems cleared up from that treatment.