This item has been identified as an outstanding known issue.
The error message "The column headers in the file you are attempting to import do not match the import definition" will appear when an import update no data in the Target Model.
This issue can be worked around by following the steps below:
If this workaround is not suitable in your use case, please contact Support atsupport@anaplan.com
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Internal Reference: CORESRV-4194