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
___
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.
___
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
___
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?
___
We've gone off the rails but I'm committed to seeing where we are going
___
___
Somehow, it keeps getting worse. I'm hopping off. This isn't even a train wreck I want to see.
___
I feel like I've been NTRed out of a wholesome story. I'll never trust again.
___
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.
___
___
___
From chapter 27 discussions.
He's using gutter oil.
___
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 
Jump in the discussion.
No email address required.
Is that even NTR? She just lost, no cheating is going on.
Jump in the discussion.
No email address required.
The genre is much broader than that. There are NTR comics which revolve around protagonist's anguish caused by his mum or sister getting fricked. NTR-slurs are subhuman.
Jump in the discussion.
No email address required.
post
Jump in the discussion.
No email address required.
More options
Context
my dad... fricked my mom...?
Jump in the discussion.
No email address required.
Hah your parents had s*x
Jump in the discussion.
No email address required.
More options
Context
You're joking but I've seen this one too.
Jump in the discussion.
No email address required.
post
Jump in the discussion.
No email address required.
MAMA〜ママを守るために僕がした事〜
Jump in the discussion.
No email address required.
based... so... fricking... based...
Jump in the discussion.
No email address required.
Jump in the discussion.
No email address required.
More options
Context
More options
Context
More options
Context
More options
Context
More options
Context
sounds like a good tragic superhero backstory
Jump in the discussion.
No email address required.
More options
Context
More options
Context
I mean I could see that as NTR if a female relative gets loudly peepeeed down by some creep just a thin wall next to your room but OP is just an interesting premise. (maybe I tolerate it because it's a foid getting cucked)
Jump in the discussion.
No email address required.
It's cute when a foid gets cucked because that brings jealousy and causes an arms race in competing for male's attention
It's disgusting when a moid gets cucked because it's sad to watch the pathetic failure of a man helplessly advance on a girl that gets piped by another dude on the regular
Jump in the discussion.
No email address required.
Also moids getting cucked tends to end with a school full of dead children
Jump in the discussion.
No email address required.
More options
Context
More options
Context
More options
Context
More options
Context
Zoomers are so pornsick that a simple love triangle is NTR to them.
Jump in the discussion.
No email address required.
It's not a love triangle if one corner is not getting any.
Jump in the discussion.
No email address required.
Jump in the discussion.
No email address required.
More options
Context
Frick that means I'm single
Jump in the discussion.
No email address required.
More options
Context
More options
Context
Some of those brainlets tried to ruin Nagatoro for me by asserting since the author also did some libertarian loli bullshit or guro or whatever automatically disqualifies them from making le wholesome highschool romance and I hit them with
Jump in the discussion.
No email address required.
Nagatoro should have been ruined on account of it being bad
Jump in the discussion.
No email address required.
Nagatoro best girl
Jump in the discussion.
No email address required.
More options
Context
More options
Context
More options
Context
More options
Context
NTR similar to the word cucked has devolved into getting lost these days but the weeb is particularly angry because they self insert into the main character
Jump in the discussion.
No email address required.
More options
Context
We don't even know if she has lost for good but anything other than a straightforward romance is ntr in the weebworld.
Jump in the discussion.
No email address required.
More options
Context
You gotta think it in meta way. Its author NTR reader.
Jump in the discussion.
No email address required.
More options
Context
I think the Japanese term is about the feeling itself, not a specific act.
Jump in the discussion.
No email address required.
More options
Context
More options
Context