Sometimes mangaka make changes between when the chapters were run in the magazine and when it's compiled into a printed volume. I've seen this happen before (where something was drastically changed). Most of the times it's minor but this case happens too.
Is there 2 different version o this manga ?? The one I read the ending and the extra are completely different (ch 11 & 12) (⊙…⊙ )
https://visortmo.com/library/manga/63341/shinyuu-to-kodzukuri-suru-koto-ni-narimashita