The condition both arises and disappears spontaneously, lasting months or years at a time. But I'm looking into what treatments might be effective other than physio (which I've booked) and simply waiting things out.
All in all, even with this diagnosis I still feel pretty fortunate. Even if it takes a long time, my shoulder will unfreeze one way or another, and I know I can adapt in the interim. Other people have gotten far worse news from their doctor, so I'm grateful.