Thanks for the info, Jacob. Good to know.
I roast indoors (no wind, temp ~22, humidity ~40) and, when this happens, it’s always before I’ve had a chance to act on the “charge” suggestion. So, the door is closed and the bean chute is plugged.
Since I can clearly see the IBTS temp, I know that it is not dipping by 80C. That leaves the ROR from the probe going to -25, which I wouldn’t see, since the graph doesn’t show negative values. (It’s pointless to stare at the Bullet’s main display and hope to see momentary excursions.)
The only thing left that could possibly cause this is an exhaust fan set-up that I have. It has a piece of 4" flex duct perched about 3 cm above the exhaust outlet of the roaster. It has a variable speed 24 VDC muffin fan in an down-stream piece of rigid aluminum duct, which is controlled by a cheap PWM controller I got on Amazon. I thought that the 3 cm gap would be enough to keep the fan from affecting the roaster temps, but maybe not.
I’ll try to come up with a larger hood-like set-up and give the roaster some more space. Maybe that will do it.