It wasn’t really like an angry twist. As you can see by this recently translated chapter, they both still have a lot of growing up to do. The mc felt like he wouldn’t be able to do this as he was relying too much on everybody else. Which leads to the breakup. At the end of the pre epilogue final chapter, he finally starts to make said progress after seeing that Hoshino has made progress dealing with her self confidence w/o makeup issue. Then the manga ends with both of them smiling as they feel satisfied with their own progress. Then the epilogue happens.
tldr; the author decided to focus heavily on the characters improving themselves (the coming of age aspect as some call it) instead of the romance. He likely planned them growing up a bit and getting together again with some build up but didn’t have time to do so or something got in the way. Hence, the epilogue.