I had my rt 700 set to full cooking some steaks and it was hitting temps of 570 turn it down to 480 and opened the lid to turn my steaks and add more steaks temp came down to 495 then started climbing back up and would not come down to 480
Opening the lid causes the temperature to drop. The controller feeds pellets to compensate. Depending on a bunch of factors it could overshoot before setting down a while later. You started on FULL so it was also cranking. The overshooting isn't unusual among pellet grills and not unique to Recteq.
Call RT if you have questions.
PS. The auger speed had nothing to do with this issue. That setting only comes into play if the grill didn't hold a low temperature in warm weather.