You are righting a story here, not a paper, yes? So I would argue the question might not be, "what pronoun should I always use for this AI?" but rather, I would say that much depends on the characters in your story, their personalities, backgrounds, beliefs, and inclinations. THEY are the ones who are going to be interacting with, and referring to, said AI, after all. Everyone is aware that the AI has no sex, of course, since it is a machine. But we refer to sexless objects by gendered pronouns all the time. A very common example would be the way ships are often referred to as "she". For a closer example, take Siri. Siri is a machine, an "it", if you will, but because the voice "sounds" female, people often say "she" and "her" when talking about Siri.
Are (non sentient) AIs common in your world, and if so, how do people usually refer to them? Do they use "he" or "she" and so on based on the AI's voice or some other arbitrary criteria (is the voice completely sexless too? If so then it would probably depend on a given person's perception). Do most people in society just refer to AIs as "it"? Or is it entirely different from person to person with no societal standard; people just call them what they choose? Whatever the "norm" is in your society will affect how many of your characters refer to your AI, whether they are aware it is sentient or not.
This is also affected by individual characters' traits. Some will fall outside the societal norm, or come from backgrounds with an entirely different norm. The way each of them thinks about the AI may vary widely. For instance, if your AI "sounds" female like Siri, and one of your characters believes it is sentient and another does not, the one who doesn't believe it may say "it" to emphasize that they think it is nothing more than an object, while the one believes it is sentient may call it "she" specifically because they find the depersonification implied by "it" to be offensive, even though they are aware that "she" isn't completely accurate either.
Finally, there is, of course, the AI itself. It's sentient, so it is entitled to an opinion here. Does it have a preference? If so, it may simply tell people how it wants to be called. In which case, maybe you can tell who actually "believes" in its sentience by who refers to it in the way it requests. Or maybe it doesn't care, (and it's very possible a sentient AI may find the whole sex and gender stuff humans are so obsessed with to be silly and it could care less) and so it lets people call it whatever as long as they aren't disrespecting it. The answer to this last part will probably help determine how you, if you write as an omniscient narrator or from the AIs perspective, refer to it in narration. When narrating from a given character's perspective, you would probably refer to the AI using the pronoun that character would use. (if you use a single narrator, it could be an interesting way to show how they view the AI. Maybe at first they aren't buying that the AI is really sentient, and their narration refers to the AI as "it" and/or other objectifying terms, but later on you can tell when they begin to believe it because there is a change in the way they think about the AI, and the pronouns used in their narration change along with that.)
