Values Not Populated in 'Last Modified Date' and 'Last Modified By' Fields

When records are inserted using the Create Audit Fields permission to preserve historical Last Modified values, certain platform behaviors can override those values after insert. Two common causes are active automation (triggers, flows, or process builders) that update the record post-insert, and the Shared Activities feature that triggers an internal update during task creation. Review the sections below to identify and address the relevant cause.

Automation

Last Modified fields may not appear to be set with inserted values if there are triggers or automation fired after the creation of records in that object. If that automation performs an update on the record, new Last Modified values will be used rather than the provided values. If the audit fields must be populated using the provided values, disable any automation that occurs after record creation.

Allow Users to Relate Multiple Contacts

When the Shared Activities feature Allow Users to Relate Multiple Contacts to Tasks and Events and the Create Audit Fields permission are both enabled, the Last Modified Date and Last Modified By fields will not reflect the inserted value. This is true if the task is associated to a contact or lead (WhoID), or other records such as Account, Opportunity, or Campaign (WhatID) during task insert.
During creation of the associated records, the task is exposed to an internal update, which results in the Last Modified Date and Last Modified By fields being updated with the current date and time as well as the user performing the task insert.
Note: The Allow Users to Relate Multiple Contacts to Tasks and Events setting is automatically enabled in Professional Edition.