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There are several possible reasons for this issue:

1. Form validation: If the submitted data does not pass the form validation rules, the form's value may be null in the database.

2. Database constraints: The database may have constraints that require certain fields to have a value. If the submitted form does not provide the required value, it may result in a null value in the database.

3. Form processing: If the form processing logic is flawed or incomplete, it is possible that the submitted form's value may be lost or overwritten.

4. Data corruption: There is also a possibility that the submitted data may have been corrupted during transmission or storage, resulting in a null value in the database.

To diagnose the issue, check the form validation rules, database constraints, and the form processing and submission logic. Additionally, validate the data transmission and storage methods to ensure that data corruption is not occurring.