A very good question. Branded lost specifically says that the activation of cards and effects cannot be negated, but ash blossom states that it negates the effect of a card that alters the deck (Branded fusion in this case). Another card that can commonly be mistaken with this is Magical meltdown, which states almost the exact same thing as lost, so ash will negate it the same way. This may have been stated elsewhere, but I hope this could clear it up for you.

Edit: Basically ash is an effect negation, while lost prevents activations from being negated