I don't think they really know what PMR is. Perhaps a combination of autoinflammatory and autoimmune.
https://pubmed.ncbi.nlm.nih.gov/34798314/#:~:text=Background%20and%20aim%3A%20Polymyalgia%20rheumatica,partially%20understood%20immune%2Dmediated%20mechanisms.
My rheumatologist made a comment to me once that made me curious. I'm not sure what disorder he was referring to because I have several. We were discussing "immune system memory." His comment was, "my immune system was unlikely to forget what it has been attacking for so many years."

I googled "immune system memory and autoimmune disorders." I found the following:

"In an autoimmune response, when memory cells are formed against the “self,” they help mount a highly efficient pathogenic response against the body's own tissues. These memory cells, by virtue of being long lived also become very difficult to eliminate."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4067599/#:~:text=In%20an%20autoimmune%20response%2C%20when,become%20very%20difficult%20to%20eliminate.
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PMR is supposed to "burn itself out" in a year or two which suggests more of an innate immune response. That clearly didn't happen in my case. The longer PMR persists, maybe it becomes an adaptive immune response.

I'm just glad an IL-6 inhibitor seems to stop my PMR symptoms. Unfortunately, my symptoms have returned whenever Actemra was stopped. This might explain why my rheumatologist has no plan to stop Actemra anytime soon. During our discussion about when to stop Actemra was when he made the "immune system memory" comment.

People with RA and other types of inflammatory arthritis don't ever have their biologics stopped unless the biologic stops working. That is usually when their immune systems form antibodies against the biologic.