Seems my PID is having trouble with the proper sintering temperature. I began the polishing by filing the imperfections out of the shank. Then when I attempted to round the ring on the mandrel, the shank split. I sawed off the broken parts to solder it back together, but the sawing exposed a hollow section with unsintered bronze powder inside. Then the ring split at the crown. End of ring.
My best guess is that the kiln is in a room where the ambient temperature is about 30 degrees lower than when the rings were sintering successfully, and the ambient temperature sensor is lowering the programmed temperature. I’ll add 30 degrees or use the AutoTune function again and see if that solves the problem.