Here is a helpful reference to osteoarthritis. https://www.arthritis.org/diseases/more-about/osteoarthritis-of-the-hands

It appears that what we are all experiencing is OA symptoms that were previously suppressed by our normal production of cortisol. If we were treated with prednisone for PMR, our adrenals stopped producing cortisol. As we taper off prednisone, our adrenals may be slow to resume cortisol production, and the decreasing prednisone dose is insufficient to control the OA discomfort. This can be expected to continue until we are off the prednisone, and we are producing sufficient cortisol again. If we never resume sufficient cortisol production, you may need to consult an endocrinologist. There are treatments other than prednisone that are preferred in that case. Best of luck on the difficult journey.