Identity Verification History stores up to 20,000 records of users' identity verification attempts for various features that require MFA, such as logging in from an unknown browser or device, resetting a password, or changing personal information. The data is stored for up to 6 months and then deleted automatically.
Therefore, if a user has been inactive for more than 6 months or if there are more than 20,000 records in total, their data may be missing from Identity Verification History. References: https://help.salesforce.com/articleView?id=sf.identity_verification_history.htm&type=5