All I can say is that the bounty set is random. Having one from a set may cause that set to come up until you clear/discard it but that sounds unlikely. As for the FoaF bounty I don't think its in game yet or (my theory) super rare from something related to ToO either the light house or gold pack maybe.
English
-
hmmm, good call, all the info around the web points to an exotic bounty which includes competing in the trials, however the mechanics of the trials have changed since then (this information has been around since launch) so it wouldn't surprise me if the requirements for getting FoaF have changed as well. I've been craving that gun since launch!!