Worked on the civicrm/civicrm-core repository to address a critical issue in event registration, focusing on participant role handling for single-registration events. Applied expertise in PHP, API development, and database management to update the API4 query logic, replacing the IN operator with CONTAINS ONE OF to accurately process serialized role data in the role_id field. This change ensured that users with multiple roles received correct notifications, resolving dev/core#5540 and reducing mis-notifications and support inquiries. The solution improved the reliability of event registration flows, aligning technical implementation with business needs for accurate communication and a smoother user experience.