I came here looking for help with the exact same issue :)
We upgraded from 8.2.3 to 8.5.1 two days ago and I have had at least a dozen complaints from users with the same issue. I have instructed then to try logging off prior to closing the file but old habits are hard to break. I will also raise a support ticket for this. Maybe if they have tickets from both of us it will get more attention.
Our users have also noticed the reappearance of an old issue. In an older version of the add-in there was an issue where you would "save as offline copy" but it would still have the formulas. This seemed to resolve itself in a more recent version but it back again with 8.5.1
Thank you
Denise
