Users may observe that activity timeline displays more records than the activity history related list for a Person Account.
This could be the result of converting a Lead to a Person Account when the activity setting ''Roll up activities to a contact's primary account' is disabled and 'Allow Users to Relate Multiple Contacts to Tasks and Events (shared activities) is enabled.
The following steps may replicate this expected behavior:
Observe that task isn't visible under the person account's activity history.
To resolve, Enable Roll Up of Activities to a Contact’s Primary Account and then edit or update the activities not visible in the activity history related list. For example, perform an edit and save or close the task.
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