I just updated the Adobe Content Viewer on my Droid X and the same thing happened, all previously downloaded folios have disappeared from the library and must be redownloaded.
Just before the ACV update I used the v18 viewer to download some updates and all connected and worked well. There's something about the v19 viewer that decided to delete the folios.
Do you know if this is still the case for the latest version?
I wonder if this is by design, or just happened with v19 update.
It would seem beneficial for Adobe to have all users need to re-download everything after an update of an app.
Hoping for a quick answer, otherwise ill have to test it myself on v21
ps, you mentioned Android, so it did not happen in iOS?