On a game against someone playing Verdant Dread, I played a Weathered Runestone to counter it, but it inconsitently prevented the cards from being manifested. It seems that the game looks at the face-up cards to determine if it can be manifested instead of treating it as a creature card as manifested cards are.

Context:

On the battlefield: One Weathered Runestone, one Verdant Dread, no other nonland permanent

Verdant Dread reads "Whenever Verdant Dread or another permanent you control named Verdant Dread enters, manifest dread. {3}{G}{G}: Conjure a card named Verdant Dread onto the battlefield. Activate only as a sorcery."

Weathered runestone reads "Nonland permanent cards in graveyards and libraries can’t enter the battlefield. Players can’t cast spells from graveyards or libraries."

Opponent activates the ability of Verdant Dread. It resolves, a conjured verdant dread goes on the bttlefield, two Verdant Dread triggers go on the stack.

Observed result:

The first trigger resolves, opponent choses a card to manifest, it appears on the battlefield as a manifested 2/2 creature.

The second trigger resolvesn opponent choses a card to manifest, Weathered runestone notifies itself and two cards go in the opponent's grveyard, no manifested creature appears on the batlefield

Expected result:

Manifest Dread rules read: "701.60a “Manifest dread” means “Look at the top two cards of your library. Manifest one of them, then put the rest into your graveyard.” See rule 701.34, “Manifest.”"

Manifest rules read: "701.34a To manifest a card, turn it face down. It becomes a 2/2 face-down creature card with no text, no name, no subtypes, and no mana cost. Put that card onto the battlefield face down. That permanent is a manifested permanent for as long as it remains face down. The effect defining its characteristics works while the card is face down and ends when it’s turned face up."

The manifested card becomes a creature card while looked at in the library, so the Weathered Runestone should see it as a creature card trying to enter the battlefield from the library and prevent it from happening, and no cards should be manifested.