Callum just responded to this other thread, like so:

Originally posted by Callum Milne:

If you attempt to manifest the top card of your library while Grafdigger's Cage is on the battlefield, nothing happens. At all. The top card of your library is completely unaffected in any way by the attempt to manifest it.

I am pretty sure that this is currently incorrect per the CR, though it is likely the intention of the rules team that it be correct. The CR for Manifest reads:

Originally posted by CR 701.32a:

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 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.

Manifest very explicitly goes out of its way to turn the card into a creature card before moving it to the battlefield. So at the very least, it momentarily becomes a creature card.

The more important issue, though, is that since the card fails to move, the effect does not end. In accordance with the bolded sentence, it is face down, and has not been turned face up. Thus, it remains a creature card face down on top of the library indefinitely, which is basically not OK – in particular, it can get shuffled, and then it's impossible to know when to stop revealing cards for an effect like Evolutionary Leap.

For this reason, this is pretty certainly a bug, and the behavior Callum describes is most likely the intended behavior.

If my understanding of all this is correct, I think a small amendment to 701.32a along the lines of replacing both instances of “face down” with “face down on the battlefield” would resolve the issue.

If my understanding is not correct, there are two possibilities:
A) Callum is correct under the CR as written. In this case, my question is: what is causing the effect to end?
B) Callum is incorrect, and it's not a bug. In this case, my question is: why is it ok to have an unknown card be a textless 2/2 creature card at a random position in the library?

The purpose of this forum is to provide {O}fficial rulings to rules questions; any ruling provided here should be taken as {O}fficial until further notice, even in cases where the Comprehensive Rules are ambiguous or provide poor support to that ruling as written. The ruling is the final authority–that is how the card is played, regardless of whether the CR properly supports that ruling.

If you believe that one of the NetReps posting answers here has made an error, or have followup questions regarding one of their rulings, you can contact that NetRep privately via email on their profile page. If they have indeed messed up (it happens to the best of us), they will correct the error immediately.

This is not one of those cases. The ruling provided in the other thread is indeed correct.