Manually downshifting while stationary in "D", will result in the transmission shifting into, and staying in 1st gear, until manually upshifting or reaching near red-line. If you're saying that manually downshifting results in the transmission going into 1st gear, even if the car is moving (in 3rd gear or higher) there's something wrong. Have you actually tried manually downshifting while driving in 3rd gear or higher?