Less "Forgotten" and more "Suppressed"
Illithid tadpoles are supposed to completely consume the host "Body and soul" erasing all aspects of them and creating a new Illithid with it's own mind and abilities in their place. This being quite literal in the sense that they eat away the hosts brain entirely and replace it while transforming their body into an Illithid one. This ensures that nothing is left of the original host and this new Illithid is completely at the command of the Nether Brain with no possibility of the original host existing again.
The tadpoles used by the Absolute are modified, in a way where they don't consume the host's mind (Even after full ceremorphosis, the Emperor still has his own mind intact - As well as whomever is relevant for the final confrontation) for whatever asinine reason, as this makes literally no sense from any perspective.
Like literally, the only thing this modification provides... Is the ability to have a bunch of gullible idiots run around as "True Souls"... Whereas if they used unmodified tadpoles, they'd just focibly implant whomever and then they'd get a completely subservient Illithid. With only extremely rare cases like Omeluum where an Illithid splits from the hivemind and develops individuality.
But I digress, the fact remains is that these (Stupidly) modified tadpoles do suppress powers, while retaining some ability to be commanded to further ceremorphosis and provide power back to the host (Which is used to entice "True Souls" by offering them power and rewarding them for furthering the Absolute's plans). However, due to the Artifact and the people within, the Absolute is cut off from providing commands to the PC/Companions tadpoles so there is no power returned after the initial suppression (As the price to prevent the later commands to instantly ceremorphose once you're actively fighting the Absolutists)
Thus all the companions are stuck as level 1 plebs until they regain power naturally (Or stick more tadpoles into their brain to unlock Illithid powers... Which is something that raises further questions... Mostly around how in the heck do multiple tadpoles co-exist peacefully inside one mind...)
