The manga in question is "It's a Little Late, but I've Fallen in Love with My Childhood Friend" (imasara desu ga osananajimi wo suki ni natte shimaimashita).
The story initially 
starts out as this extremely cutesy story of a highschool girl realising she's fallen for her childhood friend and her subsequent efforts at getting closer to said childhood friend. Now, when I say this starts out cutesy I'm not exaggerating. It's very cutesy 
and full of fluff 
. It feels like the type of manga where the MC and her crush holding hands would be treated as a major romantic development.
But come chapter 21, disaster strikes.
It is revealed that one of the side characters had been fricking our MC's object of affection 
. Yep, this is no longer a manga about two cutesy virgins getting closer to each other. Then the POV entirely shifts towards the side girl and it becomes a depressing story of how this girl is fricking our guy in order to distract herself from her fricked up family situation 
. This is a complete tonal shift from how the manga was for the past 20 or so chapters.
Suffice to say the fans were not pleased 
( as you can guess from 10x increase in comments on mangadex for chapter 21 
). The show gets bombared with negative reviews taking its ratings from an impressive 9 to a 6 with people making accounts just to shit on the author 
.
Some comments from the chapter 21 meltdown thread. :meltdown
Hey, I just said that.
Why would you introduce conflict in an already perfect wholesome series? Wtf author?
Oh wait, it's the same fricking author who did white album 2, no surprise.
Apparently, this author has done it before 
.
Bro, frick this guy
Everytime a manga post gets like 100 comments in 2 hours it's either the death of a shonen jump character, someone confessing, or problematic shit suddenly happen outta nowhere lmao
Lmao I saw the post at 3 hours and over 700 upmarseys and thought, "oh shit she must've confessed this chapter." Boy was I wrong
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The short and simple answer is no. Of course it's a common route for romance to take, though common shouldn't be conflated with good. Fricking heck it just sucks.
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Is she actually a girlfailure if the world (author) actively shits on her?
She is now a girlsurvivor
Put thru the bs of her friends and the author
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Well, was nice while it lasted, will drop it here. What a shit development. Throwing a wholesome story out the window with just an unnecessary love triangle drama. Sad to see.
Exactly. On top of it the premise didn't hint any of this. Rather the premise had been set on a short wholesome romcom. So the drama was really unexpected and this chapter is so sudden.
The seethe continues into subsequent chapters months afterwards. 
If the author wanted to write this story why did they bother with the first 20 chapters?
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We've gone off the rails but I'm committed to seeing where we are going
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Somehow, it keeps getting worse. I'm hopping off. This isn't even a train wreck I want to see.
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I feel like I've been NTRed out of a wholesome story. I'll never trust again.
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Like I've read through a lot but this tonal shift into some bootleg Boy's Abyss (surprising since Boy's Abyss was... something too) is so butt. How do you pivot something this badly.
From chapter 25 discussions 
.
She deserves better. Mangaka cooked and burned down the kitchen
Shouldve drawn hentai tbh
Normally I'd be appalled by the suggestion.
But for once I agree completely.
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From chapter 27 discussions.
He's using gutter oil.
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This has turned into a whole different story that the title don't fit anymore
___
Similar reactions on Mangadex 
.
https://mangadex.org/title/cf1c93cc-4bb5-4311-b926-0f02ad123ad2
/a/utists react
https://desuarchive.org/a/thread/269799330/#269799330
NTR is the law of the world, simple as that.
heartbreaking, praying she goes postal
I might actually pick up this manga if this happens.
Lmao
Childhoodfriends only exist to get BTFO by the new girl that comes out of nowhere
TRVKE 
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Is this foidslop or moidslop, who reads this stuff?
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i was unironically amazed at finding out that manga etc was actually a full blown genre and not just pokemon cards and dragon balls
like, adults are reading black and white bomic books
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Weebs
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i think cuckquean shit is funny actually
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Trve. I don't get how people get so upset about it.
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Animeneighbors cant even enjoy some fun drama they gotta be fully self-identifying with the main characters all the time and send authors death threats when they don't like the ending.
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Based?
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Death threats are foid behavior (weak). Real men send pipe bombs, especially at christmas when no one expects it
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its sad irl but this is a comic book lol, it's supposed to be dramatic
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Moidslop I suppose.
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Cuckslop
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Weebs are basically the straightest cute twinks in existence as they will read a manga about a guy's adventures as the best blowjob giver in Japan categorized as comedy/drama, and then rate the story based on the quality of characters and storytelling.
Jewish lives matter too foreskin bacon enthusiasts.
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