Datamart authentication blank on takeover

When you take over a dataset (or datamart in your case) the cedentials will all need to be re-entered. That is by design (and rather annoying).  Datamarts are a relatively new feature, I would not be surprised if they hadn't tested that part yet. You can raise an issue at https://issues.powerbi.com . If you have a Pro license you can consider raising a Pro ticket at https://powerbi.microsoft.com/en-us/support/pro/