If you are using a Unity batchmode build then Core-3.1.0 is not working. It uses EditorApplication.update += OnEditorUpdate; which does not trigger in batch mode. It also imports assets from an InitalizeOnLoad call which Unity does not support.

We're you able to resolve this issue?

I have the same issue, did you resolve this? iTMSTransporter could not generate an itmsp: could not find any packages bundle within the swinfo description (product-metadata.packages) The build will otherwise install succefully.