I've seen on short flights where GA will not process people from coach to First on a no show/misconnect or they will seat commuting crew into that seat. Pilots deadheading will get that seat before upgraders but not if they're commuting. FAs only if there's an open seat after all upgrades have been accommodated whether deadheading or commuting. GAs are presuming no one will complain about not getting upgraded on a sub 200 mile flight.

In no case should a working FA take a F seat unless the jump seat is inoperable. That happens from time to time.