Yes, your PIP may be backdated after mandatory reconsideration, and usually this backdating of the PIP is to the date of the original decision. There are strict time limits to asking for a mandatory reconsideration, that is, you need to ask within 1 month of the date on your decision letter, and if the DWp does change their decision, your PIP will be backdated to the date of the original decision that prompted the mandatory reconsideration.