We are currently evaluating this issue and will update this article when we have more information.
An easy workaround is to map the constituent field to the lookup id in their import file. A long term solution is to remove the first name, last name, and Lookup ID field from import (hide those fields), and they will get populated in the batch and remain read-only there. While simple this can take a long time to complete if updating a large number of records.
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