DPE Writeback Node Shows "Invalid User" and No Options Available to Change It

When opening a Data Processing Engine (DPE) definition and navigating to a Writeback node, the "User with Writeback Permission" field may display Invalid user with no selectable options in the dropdown, so the writeback user cannot be set or changed. Running a Flow or the DPE in this state fails with an error such as:

Error element [FlowActionCall]. We can't open this Data Processing Engine definition because you don't have access to the [field] field of the [object] selected in a data source node.

The picker can appear empty even when the org has active administrators. Root cause:

• The Writeback User picker only surfaces users who hold the "Manage Data Processing Engine" system permission, which in practice comes from the standard System Administrator profile plus the Data Pipelines Base User permission set license.
• If every active admin is on a cloned or custom System Administrator profile (a common hardening pattern), none of them satisfy the picker's filter and the dropdown is empty.
• System Permissions are themselves filtered by the license assigned to the permission set, so granting the permission on a permission set with the wrong license does not populate the picker.
• A previously valid selection can also read as "Invalid user" after a sandbox refresh or a user license change that removed the qualifying permission.