This item has been identified as an outstanding known issue. A workaround is provided until the issue is officially fixed.
When the import uses "column header" and contains Subset, the import will run however summary is empty and data in target cell does not get updated.
This issue can be worked around by either of following methods:
Use List Property as source item
Use "Fixed Line item" to select Subset as source
If this workaround is not suitable in your use case, please contact Support at firstname.lastname@example.org.
Internal Reference: CORESRV-3898