Our organization has merged two companies in the past. Company A merged into company B. Both companies had their own active directories and both active directories still exist. In the time since merging, the users from Company A have been migrated to Active Directory B so that all users exist on Active Directory B. However, legacy Company A users exist on both Active Directory A and Active Directory B with the same UserPrincipalName on both.

When a legacy Company A user attempts to connect to a PowerBI report, their username is returned as A\samAccountName (my guess is because A is alphabetically first. Company B users are able to access the report just fine). This is causing issues as the A\roles are not present on the report and cannot be added for various reasons.

How can we ensure that when a legacy Company A user accesses the PowerBI report/model, their userName is returned as B\samAccountName?

There is a new Azure server that solved this issue for some reports as it only interacts with the Company B active directory. That is not possible for all of our reports as some of the data in these reports exists only On Prem so we are relying on a fix that defines which Active Directory to check.