It happened with me, they sent me the card without backdating, i filled out the form and i sent it back immediately along with a letter to explain the error and i even highlighted the wrong date and pointed it out on the copy, a month later i received my card with the same mistake again!! i filed out the form and i sent it once again, my congressman was already familiar with my forever pending I-485, i informed him about this incident and he contacted the USCIS on my behalf and finally i received the card with the right date on it this time, i think it took another 3 months.
You need to fill out the form and send it with the old card, i would call the 1800# first to ask for the updated form and where to send it.
Also, make a copy of it and send it with your letter/fax to your congress man to expedite the process.
For those who don't know, Asylees get 1 year credit on their "resident since date"
due to the 1 year waiting before applying for I-485 rule.
Good luck!