Emails that are captured into Salesforce, via Einstein Activity Capture, before, or up to 24 hours after the related records are created, are not guaranteed to show in the Activity Timeline. This is working as designed.
This is due to the fact that the records need time to propagate through all back end systems before emails can properly be mapped to them. If an email is captured before the record exists, or even before the propagation has fully completed, the email will be captured, but it will not be stamped with the necessary RecordIDs to allow visibility within the system.
The propagation of new records, typically happens fairly quickly; but, if there is a backlog to work through, it can take longer which is why it is stated that it can take 'up to 24 hours'.
If you find that an email is missing from the Activity Timeline of a related record, check the following to see if it applies to the scenario in this article:
All emails captured after the related record is created, and fully processed, should show in the Activity Timeline, as expected.
If you need to have this email visible in the Activity Timeline, please submit a case to Salesforce Support. Make sure to provide the following details:
001814332

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