I did Leviathan Extreme two days ago to get a FC member through it and I didn't realise at the time that it was the HM drop until later that night.
I'm pretty sure that the Leviathan Barb does not drop from Extreme Mode considering it is used for the easier crafts while Barb of the Whorl is for harder crafts. Never before have I seen the Leviathan Barb drop from Ex mode and none of my FC nor LS members have either.
If it is a bug then something definitely needs sorting out before more people end up getting it instead of Barb of the Whorl when farming for it to save money.