Usually story like this....
After the red string gone, mc start to realised that she like the senpai. But the red string changed to someone else. It fulfills mc wishes that she like someone without string. Build up drama create urgency for the mc (if she not do anything her senpai will be taken from her). Then another drama and peak conflict then bam resolved and they got together. LOL this is just how I imagine it, not spoiler
Wait I just noticed....
On chap 52, mashiro and nagisa sleep using tshirt but when wake up they only using undershirt? Hmmm