When you start a new Mission the single lyrics you are working on are considered "Tasks", whether it requires proofreading, correcting, or completing.

Once you start a new Task it will remain in the "In progress" session for 4 hours. If you can't find your task there anymore, it will most likely be because this time-frame has passed.

After these 4 hours, you will lose your progress and the song will return to the "All tasks" list.