I think it’s because they changed the story a little from the source material to more neatly fit it into the format they needed for Season 1. To some people familiar with the source material, that’s heresy and absolutely destroyed the story. As a fellow anime-only, however, I felt the story was perfectly fine as it was told and I’m looking forward to seeing where they take it next season.
After all, it’s not like they pulled a God of High School on us.
They also tended to say the changes made it ‘hard to follow’ or ‘ruined the pacing’ or other things, but as an anime-only that just flat wasn’t true. I actually really loved the kind of elliptical feel of the story in S1.
I honestly kind of felt that source readers were border-line brigading posts about the anime on reddit. It certainly didn’t make me any more inclined to check it out!