Throughout the film we can see several advertisements of the "Joi" product: an AI that you can buy and customize. She appears with different costumes and details.
In the market scene, when K is looking at the picture of the tree and is approached by the 3 prostitutes, a version of Joi with a japanese schoolgirl outfit can be seen. Just after K's "sex" scene we can see in detail one of these advertisements with the lettering "Anything you want to see, to hear..."
During the scene in the bridge, the Joi that is talking with K is a placeholder, an empty one that can be customized after purchase, symbolized by the empty black eyes.
Her referencing him as "Joe" is just as an expression in English to talk about an arbitrary man, like in "John Doe", or "He's just a regular Joe", or "This is not easy to the average Joe". But, of course, it strikes a nerve in K, because of the personal significance of the name his Joi gave him.
Now, to speculate a bit, K's Joi choice of "Joe" as a name, albeit very personal to them, might not be so random, but a byproduct of her programming. This could be a hint to the artificiality of K's life, in having a relationship with a disembodied AI, rather than a human or another replicant, because of the immaterial aspect of their past (memories are only implants, "Joi"s don't even have implanted memories). Coming from just realizing that he is not the born person he and Joi hoped he was, added with the pain of being reminded of his relationship with his Joi, he could also be perceiving an emptiness in his own life, like this empty Joi in front of him.
Only now (to give a contrasting view with previous answer) he has the opportunity to give real meaning to his life by saving the disenfranchised replicants; and even more later, doing the same for Deckard and Stelline, by reconnecting father and daughter and giving them both a chance to be a family, in a way he could never have had.