新しい投稿でのカテゴリ選択の並び順を変更

It sounds like you have enabled the fixed category positions site setting, but not enabled the fixed category positions on create site setting. When fixed category positions on create is enabled, the category order that you have set for your categories page will be used to order categories in the composer’s category dropdown menu. There are some details about these settings here: Reorder / adjust category listings and set fixed category positions.

「いいね!」 2