Consider the following scenario:
You have a model that contains a hierarchy in Microsoft SQL Server 2012 or SQL Server 2014 Master Data Services (MDS).
You have a user who has both the model Update permission and the hierarchy root level Update permission.
You use the user to add a new member to an entity in the model in Master Data Explorer.
In this scenario, the newly added member disappears from the Master Data Explorer after the validation.
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To work around this issue, take away the hierarchy root level Update permission from the user.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.