email_response probe is showing NULL values for ROUNDTRIP metric

Document ID : KB000112488
Last Modified Date : 29/08/2018
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email_response probe is working fine for SEND metric. ROUNDTRIP metric is either missing for the same time stamps or is showing NULL values.
There is a rule on the mailbox to archive email sent by the probe for troubleshooting purposes.
The rule to archive email is set up as a "move" rule.
Changing the archive rule to "copy" allows email to stay in the inbox.  The probe is then able to read the email to process data.