Adding another data point for your reference.

I made my booking through Expedia yesterday (4 legs on QR and 1 leg on SK, with a QR ticket number that starts with 157). I provided my BA FFN during booking and I was able to select seats for free through QR's Manage My Booking (I also received confirmation email from QR).

However, this afternoon when I refreshed my booking through QR's mobile app, I saw all my selected seat were gone. When I attempted to select again, I was asked to pay.

Tonight I just chatted with QR by logging in with my QR credentials and I was to get my BA FFN added within a few minutes. After the chat, I was able to select my seats again for free.