Some of the invoices that show employee titles are displaying the incorrect titles. They do not match the titles set in the employee details.
The employee titles seen on invoices depend on the Classification set for the time entries, not the titles set at the Employees level. By default, the Time Entry Classification field takes the same value as the Title set in employee details. However, if a Classification is set at the Fee Schedules or Project Assignments level, they take priority and are reflected on the time entries, with Fee Schedules Classification getting the higher priority. So you should first check the Classification field on the time entries by making it visible in the Time Entries list view (More menu > Show/Hide Columns > Classification) and correct it there, as this is what gets finally reflected on the invoices. You can change the Classification in bulk using a Batch Update on the time entries. Check the Core Help for details on the Classification field and how to batch update the entries.