There are three different versions of Percy Jackson: Percy Jackson and the Olympians Percy, Heroes of Olympus Percy, and Trials of Apollo Percy. These are not the same character in the least, whether they share the same name or not.
One of the narrative techniques that makes Percy Jackson genuinely unique is the fact that its narrator has a distinctly different inner voice and outer voice. His inner monologues are highly opinionated, comedic, and loud, but as soon as he opens his mouth, an actual rare occurrence, he's far more serious, angry, and closed off, hardly saying a joke unless it's under his breath. Just a glance at the first chapter of the first book shows this: a long introduction filled with comedy and sarcasm, all leading up to his first line of dialogue: "I'm going to kill her." He is, like most people and especially most quiet people, a different person on the inside than he is on the outside, and we only know this because he's graciously invited us into his mind.
With the introduction of the second series in Rick Riordan's Greco-Roman mythology fantasia, Heroes of Olympus, Percy's personality started to change. This second series was no longer in first person, but in third person, with each chapter shifting into the perspective of another character, one of those characters being Percy. At first, this doesn't present a problem. Percy appears in the second book of the new series without his memories, so, for the most part, he remains quiet. He is frightened, angry at the world for doing this to him, and unsure of himself, so, naturally, his demeanor echoes his original one. Any slips in characterization can easily be waved off as a result of his personality slowly coming back to him.
By the time the third of the series comes out and Percy's memory has fully returned to him as if it never left, though, it's clear that something has gone wrong in Percy's personality. He talks often and talks loudly, and his style of speech itself morphs into a constant string of jokes and clueless comments without a hint of the dialogue ticks that made him special in the first place.
So, once I realized this, my first question was, "what happened here?"
My theory: This isn't just the first time we are seeing Percy outside of his perspective, this is the first time Rick truly saw Percy from an outside perspective, and he didn't know how to handle him. If he left Percy as he was in PJO, none of that interior thought would come through. He would be quiet, angry, and resentful, with a few rare self-deprecating jokes thrown in for good measure.
I, personally, would not be completely opposed to this. I would, obviously, prefer Percy's original narration, both because I enjoy it and because I think Rick has a real knack for first person narration, but it would be interesting to actually see the "crooked smile," the dark, brooding resting face that get him labeled a troublemaker before he speaks a word. The eight other narrators introduced in this series have the comedic bones to keep the show running. Percy can stay true to his original personality and only expose some of that inner monologue in his POV chapters.
This idea doesn't seem to have sat well with Rick, though. In an attempt to pull some of Percy's old narration into the regular story, he pulled some of Percy's inner personality onto the outside and, in doing so, created a completely different character. Of the people I've talked to about this, whether or not they realize the distance between Percy's inner thoughts and outer expression, they all know the way Percy is acting is wrong, subconsciously. It gets to a point where, when the old aspects of Percy's personality start to come back out in the fourth book, House of Hades, it can feel unnatural. Percy isn't Percy in Heroes of Olympus because he's a completely separate character of Rick's invention.
If Percy acts like this outwardly where his personality would have stopped him originally, his interior monologue must be different than it was before. If that's true, then the character we meet in HOO is not Percy at all.