Deathmist Raptor doesn't trigger when it's discarded to pay the morph cost of a creature

[Wizzerinus]

What is the expected behaviour of the card?

• Discarding Deathmist Raptor to Gathan Raiders triggers the raptor: https://magicjudge.tumblr.com/post/149415177993/if-i-discard-deathmist-raptor-to-turn-gathan

What is the actual behaviour of the card?

• Discarding Raptor to Raider does not trigger Raptor.

Screenshot

Please provide a list of affected cards Affects: [Deathmist Raptor]

Card Images

[PennyDreadfulBot]

Still bugged according to datrobino on discord.