MDM repository is not reporting why Inserts are failing. The repository was accepting new records, then later Inserts would error without an indicated reason. Updates still worked fine.
It appears that a change to the Employee model's matching rule was causing the problem. The Jaro-Winkler similarity matching rule was erroring because of missing Middle Name values in the existing golden records.
Previously the match rule used OR logic between the match conditions and did not reach the Middle Name field. The new match rule uses AND logic and so now the Middle Name fields are being evaluated, thus causing this error due to missing values.
The issue was resolved by taking a closer look at the match rules for names and changing the logic used.