I've gone over this several times. The best answer I have found is this:

If you purchased a ticket that had one flight with F and another with only one cabin then that ticket can SDC into F on both legs of a new flight pair.

If you purchased a ticket (as you did) where one leg was in F and the other in Y, you will not be able to SDC into F in both cabins, they will follow the rules for the two legs separately as if they were separate flights (even if you change cxn city).

I've gone through it both ways and been around a couple of times with agents and supervisors and that's been a consistent answer once I pushed to understand. The sense I got was the fare basis really is the guiding factor.