Upon further investigation, we have identified that while the Jenkins Plugin is expecting the "username" field in the JSON object of the payload, Bitbucket Cloud has removed the "username" field and replaced it with the Atlassian Account ID ("account_id") field as per Bitbucket Cloud REST APIs are changing to improve user privacy.

Bitbucket Cloud has removed the "username" field as per the new GDPR regulation.

To move forward with this issue, we'd like to suggest that you reach out to Jenkins to have the Plugin Updated.