This can happen when your device is running low on storage space.
Both Android and Apple devices need to have the app and all downloaded content installed to its internal device memory. When device internal memory becomes close to exhausted, you may have to reinstall some or all content after restarting.
Don't use file transfer apps to move content from one device to another.
To avoid this from happening, make sure you clear some space on the device. Keeping about 1GB of unused storage helps.
We might change the way we store files to avoid this inconvenience in the future.