Capturing last usage for users

Hi, we are using these APIs https://developer.atlassian.com/cloud/jira/platform/rest/v2/api-group-issue-search/#api-rest-api-2-search-get
To get date/time when an issue was created, updated, or commented.

https://developer.atlassian.com/cloud/jira/platform/rest/v2/api-group-audit-records/#api-rest-api-2-auditing-record-get
To get the audit log for Jira Cloud

However, noticed that initially we had large number of NULL activity and suddenly we see the activity is stamped with a date. What could be the reason for this Null activity? Also some of these users had activity but we still report it as Null.