I don't know how it is now but in pmUS the seniormost FA could boot any junior FA from whatever position the senior FA wanted to work. This could be done right up until departure (regardless of which position the FAs had originally bid). I have a feeling many senior FA wake up in the morning and think "I don't feel like doing a meal service today".
On the flip side, most new FA would probably be eager to work F because it comes with extra pay (it's a $ amount per hour, not a %, so it is a comparatively bigger amount for those who are low on the salary scale).