Futago Matomete "Kanojo" ni Shinai? - Vol. 3 Ch. 11.1 - Will you love me...?

[Freestyle]

she was shown in multiple flashbacks to have been effectively spineless, and because the ML was being ostracized by all his peers, if she stuck by his side she would be mocked/bullied by association.

She feared that most of all, and so she cowed before the bullies, and looked the other way or participated in the collective abuse toward the ML because otherwise she'd also be targeted.
For his part, the ML understood this and that's why he still doesn't hold a grudge against her to this day.
But even her behavior alongside the asshole who went to their middle school is her just being meek and cowing before him/going along with him out of fear of persecution, than it is her outright endorsing his behavior toward the ML (and the FL who was initially on that committee for the festival where they all meet/re-meet).

So I think she very much has always liked the ML. She just feared being bullied more, and made the decision that saw him ultimately leave her behind to her ongoing misery and suffering.

Fair enough but I feel like if you truly like someone you'd more likely stick by them or atleast try to make an effort somewhere to be with them, as you said she is still hesitating to this day lol.

I dont think MC has any noble reason to not hold a grudge though, since he is shown to not have much emotions he probably never placed much expectations onto her and quickly moved on basically.