It could also be that the very essence of Harry's own magic rebels against the brainwashing. Since Harry has such a unique and powerful magic within him, it might gradually start to reject the foreign and dark influence of Voldemort. This could be a slow process, with Harry experiencing flashes of his true self until finally, his magic completely overcomes the brainwashing.
One way could be through the power of love. Just like in the original series, Harry's strong bonds with his friends and his memories of love could start to break through the brainwashing. For example, a memory of his parents' sacrifice might resurface and shatter the false thoughts Voldemort implanted.
Perhaps Voldemort has some inside help. He could have bribed or threatened one of the guards or even a Ministry official who has access to Azkaban. This person could then help him get in and break Harry out. Or he might have found a secret passage into Azkaban that the Ministry doesn't even know about, and he uses that to his advantage.
One possible plot could be Voldemort brainwashing Harry to turn against his friends at Hogwarts. Harry might start acting strangely, like isolating himself from Ron and Hermione. Another plot could involve Voldemort using the brainwashed Harry to steal important magical artifacts from the school. Maybe Harry is made to believe that these artifacts are rightfully Voldemort's and that he's on a noble mission.
At first, it would be extremely tense and full of suspicion. Harry would be very wary of Voldemort's motives for protecting him. Voldemort, on the other hand, might be impatient with Harry's lack of trust.
He becomes more conflicted. Knowing he's Voldemort's son, he's constantly in turmoil about his true nature. His confidence might waver as he wonders if he's capable of the same evil as his father.
His reaction would likely be very different. At first, he might be angry that he didn't know this before. But then, he could start to see opportunities. He may think that if he can somehow control Harry as a Horcrux, he can gain more power. So, he might change his strategy from simply trying to murder Harry to trying to subdue him in a different way.
There are several ways it could happen. One way is through the power of true love. If Harry has a strong bond with someone, like his parents' love that still lives within him, it could break the hold of the love potion. Another option is that the natural magic within Harry, which has always been strong, could start to resist the potion as it goes against his true nature. Additionally, if there are any magical objects in Hogwarts, like the Mirror of Erised which has a way of showing one's true desires, it might somehow help Harry see through the false feelings induced by the love potion and break free from its effect.
Maybe Harry uses some powerful and unexpected spells. For example, he could combine the Patronus Charm with a blasting spell. The Patronus might distract Voldemort while the blasting spell hits him hard.
In this fanfiction, Voldemort's character is explored in a way that challenges our traditional view of him. His seeing Harry abused might make him reevaluate his relationship with Harry. He may start to see Harry not just as the Boy Who Lived who he wants to defeat, but as a person who has suffered, much like he might have in his own way. Harry, on the other hand, is stripped of some of his heroic facade. His abuse makes him more relatable in a sense. We can see his pain and how it affects his actions and decisions, and also how he reacts to Voldemort's new - found interest in his situation.
It might start with Voldemort's initial curiosity. He realizes Harry is more than just a threat. So he begins to observe Harry closely. As time goes by, Voldemort may start to feel a strange sense of connection, like he's the only one who truly understands Harry's power.
Well, in some fanfic scenarios, Voldemort might have made a deal with someone who has access to the Dursleys' house. For example, he could have bribed or threatened a neighbor or someone who works in the area. Then, when the time is right, he enters the house under the guise of this person's help and rescues Harry. It could also be that Voldemort has some sort of inside knowledge about the protective spells around the Dursleys' place. He finds a loophole in those spells and slips in to save Harry.