A Process Builder or Flow that includes a Record Update action targeting Tasks or Events will not perform the update on Tasks or Events that have been archived by Salesforce.
Archived Activities are Tasks and Events that Salesforce automatically moves to a read-only archive state based on age. Because archived activities are no longer returned by standard SOQL queries, any automation that relies on querying and updating these records will silently skip them.
This behavior is expected and works as designed in Salesforce.
When a Record Update action runs in Process Builder or Flow, Salesforce executes a SOQL query to retrieve the records to update. For example:
SELECT Id FROM Task WHERE ((Id = '00T4000003Acl0CEAR')) LIMIT 10001
Archived Tasks and Events are not returned by standard SOQL queries. As a result, the automation cannot find or update these records. This is a platform-level restriction, not a bug.
If you need to work with archived activities, consider the following:
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