I moved this to the Developer Portal of the Tableau Commnuity so that it gets more visibility by people who are more equipped to answer the question.
Doing some research I did confirm that the data sources are not downloaded with credentials by tabmigrate for security purposes. I found the following documentation on GitHub about this scenario and how to embed the password in an automated fashion.
I would also verify that the Tableau Server allows the embed credentials as well.
Hope this helps!