Hazoret's undying fury + lost legacy issue

Gunmix25

If you cast Hazoret's Undying Fury with a creature on the opponent's battlefield and it pulls Lost legacy to cast. The first two cards in the opponent's hand are filled with mana and the rest are discarded. The effect I see is very similar to madness, as each card is discarded the first two cards fill with mana. If another Lost legacy is cast immediately after, because of HUF's effect, the two fully charged cards are ignored and the remaining cards are discarded. A third Lost legacy will have the same result. 

I don't get this result if there are no creatures on the opponent's battlefield.


If I cast Lost Legacy from my hand instead of through HUF, the opponent's entire hand is discarded like normal. Will even discard the two loaded cards; discovered this after achieving a 5 gem swap and was able to cast a Lost Legacy from my hand in hopes to remove those two cards.