Yeah, from the League documentation on gameinfo.na.leagueoflegends.com:
Targets cannot be hit more than once by a given Ricochet attack and on-hit effects are applied only to the first target of each attack.
I think it would be reasonable to assume FoB works as an on-hit effect.
Then from Sivir's documentation:
You generate stacks of Fervor by hitting enemy champions with attacks and abilities.
and
Attacking enemy champions generates 1 stack of Fervor (2 for melee champions) and damaging enemy champions with an ability generates 2 stacks of Fervor (2 second cooldown).
If it's reproducible, sounds like a bug.