I don't know why has it worked when travelling from FAO but the logic behind is:
SB: always when purchasing XL or upfront seats (rows: A319: 1-5, 10 and 11; A320: 1-6, 12 and 13) + any seat if plus card holder or travelling on FLEXI ticket regardless of where boarding card printed/issued
Fast Track:
1) pax travelling on FLEXI ticket...
2) plus card holders (if booked seat and used pluscard)...
...regardless of seat choosen if checked in online or via app and in possession of self printed or electronic boarding card (if boarding card printed at the airport no entitlement to use Fast Track).
Hope that makes sense.