Additional cost: discard v. flashback

[Korath]

Describe the Bug:
Cards with "As an additional cost to cast this spell, discard [a specific number of cards, maybe of a specific type] [maybe at random]" are uncastable if you're casting them from somewhere other than your hand and you have only exactly enough cards to discard in your hand.

Fixture: A Mountain, an Island, and two Wastes on the bf, a Snapcaster Mage and a Swamp in your hand, and a Sonic Burst in your graveyard. Cast the Snapcaster Mage and target the Sonic Burst.
Observed: Snapcaster Mage creates an effect card with a Sonic Burst name effect attached. The Snapcaster Mage effect is unhighlighted and clicking on it does nothing. Drawing a second card, so you have two in your hand, lets you cast the Sonic Burst.

Which update are you using? (date, name)Which type? (duel, gauntlet, sealed deck)
dev 58bc91e3e.

What exactly should be the correct behavior/interaction?
Expected: Snapcaster Mage creates an effect card with a Sonic Burst name effect attached. You can cast the Sonic Burst from your graveyard with only the single Swamp in your hand.

Are any other cards possibly affected by this bug?
Casting from exile, and creating copies in exile and casting them. I don't think any of the effects that let you cast spells from your opponent's hand are in Manalink.