Until the mangaka decided to fuck it up with a pseudo-clean slate ending, with relationships apart, and characters going their own way,
including the main couple acting like they don't know each other, for reasons that, while
KIND of made sense if you ignore a lot of other factors, spit in the face of the development of the entire manga.
This outright killed the mangaka's fanbase at the time.
To make it better, the ending was retconned by getting the main couple together a few years after in-manga time, in a small omake released one month and a half after the manga, giving it the happy ending that everyone was expecting it to have.
But even that felt half-assed because the scene itself was so short, that only added to the frustration.
Of course, there's talk that the mangaka had already planned to make this omake anyway, so he didn't really "try to damage control," but it doesn't help that during that month and a half period, he got slightly confrontational on Twitter about the manga's ending.
Now, if you ask me, the manga last chapter plus the omake chapter makes the ending go from "outright garbage" to "a badly executed ending," yes, but for people who followed the manga release by release at the time, it was genuinely enraging how everything came to be.
Now, there are many speculations as to why the mangaka chose this ending because it was clear that it was going to upset a lot of people, especially if he already had the omake planned. One popular theory is that his possible anime adaptation was rejected, and he got angry, but this is nothing more than 4chan being 4chan, and nothing was ever confirmed.