Issue
The email was sent from an engagement program or email program where Recipient Time Zone was enabled, however, emails sent to several leads were not sent based on their time zone.
Solution
If the time zone calculation for the affected leads falls under the conditions mentioned in https://docs.marketo.com/display/public/DOCS/Understanding+Recipient+Time+Zone, then check in [Admin > Field Management] if the Person Time Zone field is hidden. If this field is hidden, then this would be the reason as this field is where the Person's Time Zone is stored for when it is calculated and is used to determine the Recipient Time Zone for sending emails.
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