I made a booking in PE yesterday and was able to select my seats free of charge through the app. It took one day for the seats to appear in MMB on the website. I was concerned that the selection may not stick since it did not initially show on the website, but it looked fine in the apps on both my iPhone and iPad, and all four seats looked occupied on Expert Flyer. So it's all good so far. My flights are in October so I'll periodically keep checking the seat assignment just in case.