You are currently at 9-star story and have unlocked 9-star monsters on the field.
You lowered the story quest difficulty twice, to 7-star.
You then completed the 7-star story and progressed to 8-star story.
Is the above correct?
If yes, then this is expected, because when you clear the story, it will advance the difficulty by 1 star. The monsters on the field are not affected by the difficulty of the story quests and will always be that of your highest unlocked story difficulty, which is 9-star in your case.