Responses
in the season 4 ending, on the author notes, they mentioned how they wanted to add the grandpa to the story, but ended up not doing so because of the other stuff that was going on during season 4 and how the author didn't want to add more suffering towards them....so now the author has the chance to do that now, it's this chapter when the author mentions it
https://www.mangago.me/read-manga/cherry_blossoms_after_winter/nbt/iur_chapter-ep/pg-1/
Personally I have no issues with how the plot change in this season but I think the issue is that previous season there is no mention of the grandfather at all and how he is except a small part. It would have been better if in the previous seasons the author could show bits and pieces of the grandfather and how he collected infos of his grandson and show how he was not happy but also show how the grandmother keep track of him, like just a prologue for this seasons. If that happen then I think this seasons would make so much sense. Showing that after the grandmother died the grandfather starts moving on with his plan to control teasung