So, there's this thing that authors do, whether they know it or not, when they start writing a story. The first few chapters the author makes a "promise" with its reader and that promise is "I've set an expectation for what you can expect to see in this story from here on out" meaning if your first few chapters are about a realistic school fight club type of deal you stick with that premise but if on chapter 40 you suddenly have characters go full on supernatural abilities without properly building the foundation for that transition you give your readers narrative whiplash and this is the feeling I'm getting from this story.
Chapters 1-12 have been more on the fluffy side of things and generally gave off a whimsical type of feeling and on chapter 12 we had a typical fluffy getting together chapter. Then on chapter 13 we're smacked with "oh shit he pumped and dumped" and chapter 14 it's "even though we have sex I'm insecure about my feelings."
It genuinely feels like the author went "Okay, now that I practice writing a fluffy lovestory lets try writing about more serious things" but instead of creating another story to fit that type of style (even the art-style used, specifically the panel and text placement towards the end, in chapter 14 feels like it belongs in another manga since that style is basically a staple in drama heavy mangas,) the author just shoehorns this different style into a manga that is basically the opposite.
The building blocks to transition into a more drama heavy story were there but the author never fleshed it out so now that we're transitioning into the drama heavy part of the story it's giving people narrative whiplash due to the promise being broken.