I disagree. She was completely valid for hating on the bimbo and her 3 himbos.
I'ts not about them falling out of love, it's about the circumstances when it happened.
They just came back from the travel of killing the demon king where she offered herself as a decoy and had been bedridden for months since. Instead of worrying for her and checking out on her they were partying and following this rando.
Fine, let's say they were overwhelmed by her beauty and wits enough to forget about their last love. Aren't they still friends? Shouldn't they still care about her well being? Check if she is, you know, ALIVE? And then when she wakes up, instead of being happy they think: good! now I can get my stuff back and give it to my new chick! :)
This, at the very least makes them scumbags. I'ts completely fine to fall out of love, but the when they did it, how they did it, and how fast they did it is what makes her reaction 100% valid.
Also, even before meeting grace and confirming she is a bi**; she supposedly went to a banquet to celebrate and thank the return of the 4 heroes. let me repeat because there is no caps for numbers 4!!! heroes, but one couldn't attend because she was BERRIDEN (and we are judging this from selonias pov, not knowing she caused all of this and she is doing it on purpose, just to see if she is justified to be mad with 0 context).
Grace, who claims to be there to honor them like the rest, cozies up to HER FIANCE AND THE OTHER 2. Even if it was unintentionally, he doesn't refuse them, and parades them over the parties.
I mean, COM ON!!!

Cause I'm lazy and don't feel like typing out too much don't expect a proper "review"
I don't fault FL for not returning the feelings of her two friends. Yes, when she first hated on Grace for taking the affections of her 2 friends - it did feel weird but the reason I could continue reading was because SHE came to a point where she said, "I'd understand if they fell out of love with me. I won't force them" (or something like that).
Honestly thought she was a good sport, she only got suspicious cause they all left her at the same time. Honestly who wouldn't AND she made it a point to not lead them on. She clearly says that it a flashback ("I took the hand of my fiance not to lead them on" - again I stand to be corrected). Her treatment of the MLs (not TAN) is fair because ALTOUGH us (and FL eventually) knows that they are being brainwashed- the treatment she is currently receiving in their state is well deserved. I'd rather have her fight and eventually dislike them. She never claimed to be a saint.
I do still feel that the behavior the MLs ( not TAN) is showing feels a bit forced and over the top. Like the author needed them to be hated so we could be more supportive of the couple. I get it but I don't like it.
I probably couldn't write a better story cause I'm lazy but I'll come back when the villainess is defeated.
P.S: Love her BTW. I hate her for the plot but the scheming is satisfying. Love it when villains are villains for nonsensical reasons cause why be good and boring
P.P.S: LOL( ̄∇ ̄") i said I was lazy too type but got carried away. It's still not all that I wanted to say but I guess yall get the gist of it.
P.P.P.S: the father's betrayal hurt me so badly