A reason for this could be that your offline.app file is not digitally signed and MacOS requires that all .app files are digitally signed in order to run. Similar windows 8, 8.1 and 10 have the same requirement.
Why isn't the .app or .exe signed?
3D Issue unfortunately does not sign the outputted files as this would require the distribution of our code signing certificate and keys with the software. Which would pose a serious security risk to our users and their readers.
To sign your offline files you would first need to purchase a code certificate from a code certificate provider (such as Comodo, DigiCert, etc.).
You could then use your certificate to sign your files using a codesign tool. Below are some examples from Digicert.