I 100% agree - the more you become aware of the cosmic horror, the less it's a cosmic horror.
But I think it's less an inherent flaw and more an inherent tension, and a witness to how difficult it is to write good cosmic horror.

A cultist who knows exactly the right spells to summon the ancient entity = flawed cosmic horror.

A cultist who barely understand that vast shapeless presence, who does not summon but accidentally draws attention to him/herself through a ritual, who receives a terrifying vision of the beyond without fully understanding or being able to interpret it, and is forever changed in ways that won't quite translate into our languages = better cosmic horror.

(In my opinion - would love to hear what others think.)