This is a bug report.

Pearl Lake Warden states that it may be cast from the top of the library, but when it is on the top of the library it is not visible; the only time it becomes visible is if it is put under with Scry effects.

The card is an 'Arena-only' card, and the functionality of revealing itself when it is on the top of the deck is a unique feature for Arena. It's sad that it doesn't work properly as cards that let you cast them before you draw them are an interesting design space. I could see things like a bolt that lets you cast it for (R) if it's the top card of the deck.

However, you have to have those cards show themself when they're the top card of the deck; I even played one game to test this bug and clicked on the top card of my deck every time for three games until I was certain I had clicked on the Warden when it was on the top of the deck.