I think the author just realised that the original concept wasn't gonna move the plot forward, but this will. I don't think having the plot worked out far in advance is that important actually
Idk about you but I can't read something something that wasn't tought out from the start, the whole reason the concept of love square is interesting is the promise of a reveal and how the characters will handle it.
Definitely ultra slow burn
Honestly, I think only good writers can pull off stories that are improvised, but I don't believe improvisation is antithetical to quality. If it's well written you can't tell it wasn't planned. Not talking about this story in particular though lol
I don't feel good about that last minute plot change, kinda tells you the author doesn't plan ahead and just writes with the flow, I'd be suprised if they didn't. there's an overwhelming number of side characters too who serve no purpose to the story. Just when I started thinking there was going to be progress and Hanae would confess, they hit me with this (very likely) miscommunication arc.
I feel like I jinxed this manga by saying it would see a reveal before miraculous ladybug. I guess this is the fate of all these love square media.(A bit like how every yandersim-ish game is bound to doom)