When Frontier changed the look of the profile page, the ability to select a seat preference was removed. I can say that my preference in my "old" profile page was for an aisle seat; I did not specify front, middle, or back of the airplane, as the only thing I cared about was the aisle seat. Since the new profile page debuted, the few flights I have taken have all given me window seats, save for one flight which DID give me an aisle.
So in other words, it appears that the old preferences are not being recognized or "remembered".