In summary, the transguy character detransitions to a cisgirl.
I'm sure you can see where my grievances already lie, but I must make a few things clear first. Detransitioning is perfectly acceptable. No one should feel guilty or feel their experience as trans was now "invalid" or "fake" because they "returned" to identifying as the gender they were assigned at birth. Gender is a fluid, ever-changing concept, and you have every right to be whoever you feel like at any time.
Detransitioning isn't the problem. Gender fluidity isn't the problem. The problem is that this is the one sole transman character in the entire story.
Well-written and represented trans characters are hard to come by as is; transman characters are nigh-impossible. If you come across one in a story, chances are it's simply masquerading as an "actually cisgirl trying to escape the patriarchal society by presenting as a man" or an "actually an insecure cisgirl in denial of their femininity" plot. This isn't to say people like this don't exist in real life, or that they should in any way be ashamed for having faced said struggles and ultimately embraced their femininity. But that's the only narrative that's ever offered. The possibility that they genuinely want to be a man or a butch woman seems to never, ever be an acceptable option.
This hurts a lot. And it hurts more precisely because Hourou Musuko is such a well-written story, one encompassing 123 chapters, that captured the hearts of so many readers, trans, non-binary, and cis alike. That its one initially-transman character is given the weight, realism, and sympathy they deserve, also makes their eventual culmination in them ultimately deciding to identify with their assigned gender and change their appearance to fit society's norms (for some reason, the idea that the character may just want to be a masculine-presenting woman is out of the question too) hurt all the more as well.
If Hourou Musuko had more than one transman character in its story, this would be acceptable. If Hourou Musuko explored the intricacies and struggles of detransitioning, this would be acceptable.
But it does neither and, in the end, it's just a giant letdown.