Looking at the offline capabilities in section 2 and 3 here, it doesn't mention encryption at all. It does mention the following:
“ Data is stored in a local SQLite database, which is only accessible by the app. ”
However, looking at SQLite databases, it doesn't come with encryption by default, although there are options available for it. Based on no explicit mention of encryption, and the default of no encryption of the SQLite database, I would assume that the data on the device is not encrypted.
As far as I know the sqllite database is indeed not encrypted.
If the deviced is managed you could leverage data encryption policies on OS level.
Or alternatively you could resort to an app wrapping solution offering app level encryption.