Mayhem doesn't interact correctly with Madness

[Grath]

Cards discarded into exile with Madness then moved to the graveyard are still considered discarded cards:

702.35c After resolving a madness triggered ability, if the exiled card wasn’t cast and was moved to a public zone, effects referencing the discarded card can find that card. See rule 400.7k.

However they do not show up as castable with Mayhem.