Prednisone adversely affects collagen synthesis.

Tendons are composed of collagen fibers. Collagen is the primary building block of your body’s skin, muscles, bones, tendons, ligaments, cartilage,and other connective tissues. Prednisone has an adverse effect on all of these structures. Prednisone's effect on skin, muscles and bones are more obvious than tendons.

While Prednisone does relieve the pain caused by inflammation --- it should be noted that inflammation is a normal process for the repair of connective tissues like tendons. Reparative inflammation is a good thing. PMR inflammation is maladaptive inflammation and not a good thing.
https://pmc.ncbi.nlm.nih.gov/articles/PMC8327090/
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Healthy patients with no PMR who are exposed to chronic corticosteroids still have degradation of collagen. I believe PMR with its chronic inflammation contributes to the collagen problem caused by prednisone.

For what it is worth ... medical problems are always multifactorial ... it is never only one thing that causes a problem.