Okay, this isn’t exactly an inconsistency, but it is an unfair accusation all the same. People like to give Akane grief over her inability to recognize that P-Chan is Ryoga; however, I’d argue there are plenty of reasons to justify why she doesn’t make that connection.
Look at Ranma and the Jusenkyo guide, for example. The Jusenkyo guide WORKS at Jusenkyo and knows the hazards of the springs, yet when he’s presented with a tiny black piglet and jokes about the possibility of him being a victim of the spring – despite Ryoga clearly reacting, nodding, and responding to what he’s saying – he proceeds to boil him alive rather than check to make sure he’s genuinely a pig first. Even the "expert" couldn't tell. And then there’s Ranma. Ranma, who, after realizing Ryoga has a curse, believes he’s a dumb, docile dog – not the angry, bandana-clad piglet who despises Ranma on sight, blushes around a girl, is univocally male, and who Akane just happened to discover in her bedroom after Ryoga fled. And even after realizing the dog isn’t Ryoga, Ranma STILL doesn’t make the connection until after he submerges the pig in hot water and he sees Ryoga transform before his eyes. Who knows how long it would’ve taken him to make the connection if Akane hadn’t insisted that he bathe her pet.
In fact, they're not alone. Even Ryoga didn’t make the connection that Ranma and Genma were responsible for his misery in China, despite seeing a SLEEPING PANDA next to Ranma. It wasn’t until Ryoga saw them together in the bath that it all clicked into place, and even then, Ranma didn't make the connection right away, either, despite sitting there naked as a girl. Both boys are oblivious when the story calls for it.
And unlike them (and all the other characters who know his secret), Akane has yet to see Ryoga transform for herself. And she’s not the only one. Here are all the people who DON’T know Ryoga is P-Chan: Akane, Kasumi, Nabiki, Happosai, Ukyo, Kuno, Kodachi, Principle Kuno, Miss Hinako, and all the Furinkan students.
So, honestly, ALL these characters are dumb and oblivious when it suits the story. Akane is no exception.
One of the main reasons I think Akane doesn’t suspect Ryoga is P-Chan is the most obvious: Nobody has told her the truth. Sure, Ranma teases and drops hints, but since he’s rarely honest with her and likes to tease Ryoga, it’s not surprising she doesn’t take his insults seriously, especially since he could easily expose the pervert by pouring hot water on Ryoga at any time. Frankly, if Ryoga were another Jusenkyo victim, by her logic, then SOMEONE would have told her already. Genma, Shampoo, Mousse, HER FATHER . . . honestly, none of them have good reason to keep Ryoga’s secret, and Shampoo has every reason in the world to drop that bombshell. Yet, for some reason, nobody is willing to take the plunge.
Also, as shown repeatedly, Jusenkyo victims are water magnets who constantly get wet, forcing them to transform at inconvenient times. Being around them as often as she has, Akane has seen the Jusenkyo hoard transform constantly, so if Ryoga were a victim too, wouldn't she have seen him transform by now? Looking at it from her perspective: Ryoga doesn't avoid water at all. He fought Ranma in a giant pool in the ice skating arc, took her on a "date" to the lake, participated in the hot-water obstacle course race, helped rescue her on a mountain rigged with water, and she's even seen him get wet, etc. So for someone supposedly guarding such a big secret, why would he leave himself open to discovery like that? In her mind, she likely forgot the context of all those incidents and convinced herself it wasn't possible; but memory is a fickle thing that way. So taking all of that into account, the simple fact is that Ryoga has had some INCREDIBLE luck. Is it any wonder she never took Ranma's idle comments seriously?
And then there’s her character to consider: Akane is shown to be nice and trusting by nature. Countless people have tried to hurt, kidnap, forcefully date, and betray her - Shampoo, Kuno, Pantyhose Taro, Gosunkugi, Principle Kuno, etc. – yet she always forgives them and treats them respectfully and kindly. So why would Ryoga be any different? To her, he’s a nice, thoughtful person. A friend. She has no reason to suspect he’s been betraying her trust in such a terrible way. The thought is inconceivable to her.
People tend to ignore the obvious: Akane can’t learn about P-Chan because it’s a valuable storytelling tool. It creates humor, misunderstandings, and jealousy, all of which help move the story along, particularly the romance. Ranma’s jealousy over Ryoga is a driving force in their love story and one of the consistent ways Rumiko Takahashi can show that Ranma’s feelings towards Akane might not be as ambivalent as he claims. Revealing “the truth” would irrevocably destroy things and put a messy spin on several key relationships, changing the tone from comedic to overly dramatic. At least for a time.
Besides, when you remove the “Ranma is a comedy” mindset and look closely at the P-Chan situation, what Ryoga’s doing is anything but funny. It’s disgusting and morally reprehensible. That’s probably the main reason Rumiko Takahashi never resolved it and never planned to: The fall-out would be too massive, not only for Akane and Ryoga but also for Ranma and Akane. Akane may be a forgiving person, but there’s no way she would place the blame solely on Ryoga’s shoulders, even if that’s where it technically belongs. Knowing her personality and her hatred for perverts, as well as Ryoga's penchant for getting lost, she'll need a physical target to take her anger out on, and in a way, Ranma is as guilty as Ryoga.
Since she loves and trusts him, the betrayal will hit harder.
I think Ranma VASTLY underestimates his part in all of this. Despite his justifications, he chose to keep Ryoga’s secret, and ultimately, he put Ryoga’s honor above Akane's. As her fiancé, I don’t think that’s a betrayal she could easily forgive, whether he was honoring "the warrior's code" or not. To put it bluntly, Ranma let another man secretly watch her undress, cuddle, nuzzle, and sleep with her. For someone with a pervert-complex, that's not something a simple “sorry” or "I should have told you sooner" can fix. Not without lots of anger and tears first. And Ranma isn't the type to apologize or admit fault anyway, so I doubt it's something that would resolve itself easily.
So I don't fault Rumiko Takahashi at all for choosing never to address it either. Like her, I'd let these characters take that secret with them to the grave LOL