When migrating from SharePoint Online to SharePoint Online, Modern Pages Thumbnail is not preserved. This results in Modern WebParts not displaying the correct picture.
Our development team is aware of this behavior and is working towards a solution in a future release of Essentials for Office 365.
As a temporary workaround, the following steps may be followed:
1. Open the affected page and click "Edit"
2. Click on "Page Details"
3. To update the Page Thumbnail click on "Change thumbnail"
Detailed information about editing pages details is available here:
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