The below information is applicable to continuous flow pans (AKA divided pans). If you are using a flat pan, you should NOT draw off a considerable quantity of syrup while the pans are under fire.
It is very common for the temperature to rise about 2ºF above the set point on the first draw off of the day due to some mixing that may have occurred since your last boiling day. The temperature may rise slightly during subsequent draw offs too, but not as much. If the syrup temperature consistently rises too high for your preferences, start drawing off at a lower temperature.