I just picked up on mirroring scenes that may shed some light on the K and Joi relationship.
In the original Blade Runner Deckard is presented as a human and though there are subtle and not so subtle hints that he may be a replicant in the film they don't really matter because Deckard believes himself to be human.
Deckard the human or Deckard the replicant who believes he is a human unquestionably falls in love with Rachel whom Deckard knows is a replicant.
An authentic human falls in love with a synthetic human.
2049 poignantly illustrates this by having Wallace try to entice Deckard by parading out a replicant that looks exactly like Rachel, only it isn't Rachel.
"Her eyes were Green" - Deckard
It wasn't just the eye color that was off though. Deckard knew that the Rachel impostor replicant wasn't his Rachel despite being momentarily stunned at seeing the visage of his lost love standing before him.
She was beautiful and familiar yes but also hollow and devoid of the memories and experiences that Deckard and Rachel shared together that made them "real" despite being synthethic.
It's hard to know for sure if Joi's relationship with K made her authentic or if everything she did for K was just part of her programming to make a lonely guy feel special/loved.
What is obvious though is that K being a replicant fell in love with an AI hologram which mirrors Deckard the human falling in love with Rachel the synthetic human.
The scene where K see's the large nude holographic ad for Joi is a mirror of Deckard being confronted with the Rachel impostor .
K see's the nude Joi ad for what it is. It's a program designed to be a companion for the lonely and lustful. It is hollow and an impostor of his Joi of whom he shared memories and experiences with that made her real to him.
I think all this can be summed up by a seemingly throwaway line from Deckard in 2049 that stuck with me.
When K asks Deckard if his dog is "real" Deckard responds "I don't know why don't you ask him".