I've raised an request for information ticket with support and they answered the following.
When an error occurs on an offline device, the error will be logged in the console. You're able to see the messages when you debug the application. The log messages aren't persisted in a log file, so when you don't catch them in the console, they will be gone.
For more information on debugging a mobile app, see https://docs.mendix.com/howto/mobile/debug-a-mobile-app
unfortunately, there's no way to find details of what is going wrong on the device without actively debugging the application. There might be clues in the application log, for example when the database isn't available, causing a synchronization to fail, which will result in an error in the client, but this isn't always the case.