omfg, thank god I'm not the only one that hates this trope. It turns the fmc into a trophy and the majority of the time they have no say in the matter. This is marginally better since she goes along with it but I hate it all the same.
Idk about those types of heroines but 90% of the time, the first girl in a harem is the one to win. Unless they go all in on the harem aspect like 100 girlfriends, a couple of cuckoos or Quintessential Quintuplets.
I didn't know that it is Pixiv, it could be the author getting bored of the series so they want to wrap it up quickly. A personal axing compared to the regular type.