Join Date: Oct 2002
Location: Here there everywhere
Programs: Bonvoy Lifetime Titanium, Hilton Diamond, IHG Plat, BA Silver, Aegean Gold, Aeroplan 25k, AA EXP
Posts: 2,822
CO's rule is platinum access is granted if flying on a trip including international sectors.
In this case you should have gained access. Lounge access for FB Gold members is more of a grey area with variations every week it seems.
I'd send an email to Flying Blue complaining, you might get some free miles as some have had here before.