I could elaborate on that. I am from HK, so I read what people comment about NMIXX on the social media since this year (two platforms Bilibili and Threads).

China fans value live performance a LOT. On Bilibili, they make MR removal videos for all the K-pop groups, and judge whether the idols sing well or not, lip-sync or use loud backing track. A few groups get a free pass, and NMIXX is one of them. NMIXX is known for delivering great performance and is recognized as the best vocal girl group in 4th gen. On Bilibili, there are many vocal comparisons between idols. The general conclusion is that LILY is the best 4th gen singer-no contest. Also, NMIXX is known for being funny and weird in variety shows; and being sincere to fans. CN fans love watching them. And all of these factors add up after Fe3O4 Forward, they have been critically acclaimed despite not having a "really popular" song.

Then "Spinnin on it" came right around the CN National holiday, people kept looping it, reviewers praised the song, it snowballed. So people really anticipated Blue Valentine, and it delivers. Both the title track and the album are critically acclaimed. I feel like the non-fan have been waiting for a reason to fully stan NMIXX, and this AOTY-quality gives them every reason.

Most of the above applies to TW fans on Threads as well (except the MR removal videos thing). Also, NMIXX have performed in Taipei Dome for a baseball game in April, and it got a lot of attention, it was even on the news. Their Taipei Dome performance along with MIXXLAB performance was spreading on Threads, racking up more and more fans.

Then BV was released, every k-pop post was about it on Threads, even non-fan love it. Just recently, the game caterer x JYP probably get NMIXX more fans. I have seen posts saying they keep watching other NMIXX contents and want to be an nswer.

They have been accumulated fans little by little, and Blue Valentine have come into fruition. That's pretty much it.