You may have noticed a new button within your Reviews.
The wrap up button allows you to finish up a review session in a way that saves all progress.
Previously if you wanted to end your reviews, you would just exit out of the session. The problem with this is if you had pending partially answered items, exiting would make it so these items aren’t updated and you would have to review them again 
So, instead of losing your progress, just click on the Wrap up button. This will allow you to finish up all partially reviewed items.
We’ve updated the docs for future reference.
Happy studying!