Not sure what is meant by 40°F spread between IBTS and the drum temp - they’re the same measurement. Did you mean drum temp vs. bean temp? That would be comparing the IBTS (drum temp) to the thermocouple (bean temp) mounted on the inside of the face plate. Depending on where I am in the roast, and with a clean IBTS, I see about 50F° difference after RoR starts registering. That difference shrinks by about 50% over ~2 min period after start of 1st crack and stays pretty constant into the start of 2nd crack.
In the past I’ve had the impression IBTS is pretty accurate when the sensor lens is clean. After a dozen or more roasts the difference between IBTS/drum temp and bean temp would diminish indicating the IBTS lens was getting dirty. It turned out that for the first 11 months of using the Bullet I had a defective IBTS fan (the related ErC was introduced in Nov 2019 with FW 540), so I’m not sure anymore how quickly the IBTS lens will get dirty now that it has proper air flow.
Btw, bean temp readings at rest (pre-charge) are less before starting the first roast than what you will see at rest after the first roast. The face plate & thermocouple don’t get heat-soaked without beans in the drum. Bean temp readings are pretty consistent after the first roast of the day.