Weird. I assume you aren't accidentally knocking the gear lever over to the left (manual) of drive when selecting.
I just wondered about the paddles because, as you probably know, if you shift with paddles in Drive, it goes into manual but if you then don't touch the paddles for a short time it reverts back to drive mode, or if you keep your finger held down on the + paddle it also puts it back into drive mode. Just wondered if you may inadvertently have done this.