Fluctuating temperature

Earlier today, I clarified some butter in my APO. (Hat tip to Ragnhild for the advice.) The recipe calls for running 185F (85C) 0% steam for 30 minutes. I set the APO to SVM 185F 0% Rear and started it. Once it reached 185F, it kept rising all the way to 213.5F (thought SVM was limited to 212F). It stayed up there for a while and then all of a sudden plummeted in less than a minute down to 154F. It then climbed back up above 200F, held for a while and plummeted again. It never came close to stabilizing. I managed to finish the clarified butter and then ran a few tests. The temperature never came close to stabilizing during SVM with 0% steam at 150F or 185F. It did much better at both temperatures with 100% steam. It was also very stable at 325F 0%.

What is puzzling is that I roasted a chicken based on Scott’s Roast Chicken 101 a few days ago and didn’t notice this problem. There are recipes in the Anova app that call for low temps with 0% steam, so it should be possible to achieve a stable temperature at those settings. Is this a problem with the wet bulb sensor? Does it need to be primed if cooking without steam?