Summary / TL:DR: This article outlines a known issue with pre-headers not populating in the Windows Outlook client when sending a campaign using a Sparkpost adapter.
This has only been observed with the Windows Outlook client.
Linked Issues
WTSM-216338 - Getting issue details... STATUS
WTSM-201962 - Getting issue details... STATUS
Issue
Currently, when sending a campaign using a Sparkpost adapter and the email creative contains a pre-header, the pre-header does not appear when using the Windows Outlook client.
However, the pre-header will populate in other email platforms like Gmail, Yahoo, MacOS Outlook Client, etc.
Microsoft forum that outlines the issue: https://answers.microsoft.com/en-us/outlook_com/forum/all/preview-preheader-text-missing-bug/03c560fb-a00c-44c1-8b33-42bc519d3373