Talk:The Garden of Sinners Collaboration Event (US)/@comment-2601:5C1:4401:9B53:20E8:D684:22E4:F601-20180207163131/@comment-28209103-20180207164046

even if you haven't unlocked the quest yet the game will keep track of your progress

so if you've finished the 6th, 7th and 8th but have yet completed floor 5th, then once you unlock it those mission will be flagged complete.

in case you still doesn't get it, some progressive quest usually have the requirement of finishing the previous quest to unlock. mission 73-80 counted as progressive quest.