Remember that a DIFFERENCE can happen from Novel to Manga! Like the Manga changing or taking something out PRECISLY because of what you said, what I illustrated
I mean I forgot the name of the manga, but it was about a guy who fell to despair cause his Saintess Childhood friend/fiancee went off to defeat the Demon Lord with the Hero who doesn't hide he plans to steal her from him. And when they get back, it's annouched they will be married! In the Novel version of it, she really DID betray him for the Hero! While the Manga instead revealed the Hero and church had LIED about the Engagement before the Saintess could make things clear and MC in despair ran off without confirming it with her first! She spend the story pretty much trying to find him to tell him she had NOT betrayed him! Of course in a sense this DOES undermine the HAREM aspect from the Novel as now the reader might be invested in her
Then there was that Dark Knight Isekai where the Protagonist's childhood friend betrayed him for the asshole hero, while having a sense of entitlement to him as she doesn't like his new lover! The LN changed her to instead be a NOD to the "Dense Hero Protagonists" who don't realize they have a love interest! Namely she is UNAWARE the HEro Loser NOR MC have feelings for her (Hero Loser is someone she respects and MC is like a little brother to her! She just never made these CLEAR so to the Loser, he think he has a chance to WIN HER to his harem, while to MC he thinks "she chose" the loser! Hell the asshole's harem is actually TRYING to get her to ACKNOWLEDGE her feelings to MC AND she does have a JUSTIFED reason to dislike MC lover as MC is teh DARK KNIGHT fighing for the "dark god" she believed the Goddess lie that he's being mind control and the Lover is just a servant meant to control MC! In the story, the Dark Gods aren't evil! It's more the LIGHT GODS minus the head are narcississitic Lookist assholes, and the Goddess in question REFUSES to marry the Dark God so summoned the Loser and his harem as disposables to get rid of him! MC was summoned by the dark god as a counter and helps he HATES the loser hero for "Stealing" his childhood friend, he didn't steal her! The girl was bodyguarding him because of their clan's orders! MC just took losing a match against him as PROOF he lost her to the asshole when the match didn't matter and the Asshole wasn't a MASTER like the MC thought he was, he was a lazy genius who can WIN if he catches his foe off guard because he's "Unskilled but strong" but if he FACED those same foes AGAIN now AWARE of how he fights, he's a chump EASILY defeated as MC showed, he was scared of him at first BECAUSE of his lack of confidence and the first lost back on Earth, but he had trained hoping for a rematch, and it was now PAYING ATTENTION since he OBSESSED over that lost that he di. Honestlty the change on HER was a GOOD MOVE because her WN self was UNLIKEABLE compared to her LN self who is FAR more likable because it's not a SENSE OF OWNERSHIP on MC but more GENINUELY wanting to save him and ironically acting MORE the HERO then the Loser (The goddess fell in love with MC at that, her motivation for using the loser changed to instead "freeing MC from the Dark God's clutches/side by claiming brainwashing to replace loser as her NEW HERO/Consort!" It's funny cause MC LOVER is actually a CLONE she's connected too! It's Called "Ankoku Kishi Monogatari" and it's criminal it's still in Novel form! Check it's entry on Tvtropes.org for more! if you're interested. Found that one alongside "Death Mage doesn't want a 4th" a decade back!
But largely back to this story! As you've seen, the STBX have ALOT of issues when it comes to LOYALITY! MC STBX is the LESS evil as she is mostly just controlled by her LOINS and the THRILL of the cheating! It's like a DRUG to her that she can't quit! The FMC STBX is a playboy asshole who IS aware MC STBX IS someone's girlfriend and gets a thrill out of "Stealing her" like a creep! Both assume their respective BF/GF are unaware but as the point of the story, they ARE delusional in thinking they can just "go back to pretending" especially when they didn't even TRY that hard to HIDE that it's the CAUSE OF THE STORY IN THE FIRST PLACE (FMC first became suspicious, then MC RIGHT as she was getting around to telling him that her BF was cheating with his GF)
MC STBX gets caught off guard when MC tries to end things with her, ESPECIALLY after her "habit" for THRILLS caught up to her! at last and she refuses to accept she was caught, ESPECIALLY as being pregnant and the ASSHOLE refusing to accept responsibility means she has to CLING to MC otherwise she's raising that child ALONE! Getting caught NOW is the WORSE for her in that selfish regard, but I think the MC chose NOT to reveal on her and let her walk away peacefully till she kept PUSHING and got their families involved. It's why I said I think the writer as in the NOVEL might have only added the "mom was a hoe too" as a genetic jab at the Ex!" And it made me PRESUME her mother ALSO did what she was trying to do to MC onto her father, meaning SHE was an AFFAIR BABY! I don't like this largely cause I HATE that Genetics thing, I got buddies who are AFFAIR babies themselves who wouldn't DREAM of cheating on their lovers/SO's And i'm the SON of a UNREPENTANT Serial Cheater and I hate Cheating IN GENERAL long before knowing my Dad was a Serial Cheater and it's a BIG "tension point" between me and him! Largely cause he put me in the middle of it out of laziness! It was his weekend with me, and instead of CHANGING TIMES with his other woman! He chose to take me ALONG and I WASN'T HAPPY! My mom WARNED ME on this when I was 13, this happened when I was 17 and Guess what since 10 as dad LEFT and had no contact with Ma and me when I was 1 till then! I knew his GF who was like an AUNT to me, so imagine showing someone like me THIS display, and worse the woman was unaware he was with someone too! Only reason I didn't snitch was my mother, she was PISSED he put me in the middle of that shit and TOLD ME to say nothing! At least at 18 both women DUMPED his ass on their own and did better! But still I hate myself for doing as my Mom said even if she was looking out for me! And it's a SORE SPOT with me and the old man that he would even MAKE ME AWARE OF THAT! Especially as SEVEN YEARS he was actually looking GOOD in my eyes just to BLOW IT!
I think the Manga is trying something with her, hence why I can see what you mean by "doesn't fit the tone!" Sometimes transition from Novel to Manga makes the writers ADD stuff before realizing TOO LATE that it's caused OTHER PLOT POINTS to be difficult to take note off! This is also cause the Novels usually go "POV" for chapters where Manga is likely to give MULTIPLE attention! Ex likely didn't get as much attention as the Manga is giving her! So tones feel off /