I just had the same problem when sharing an album pushed to the cloud from a local folder on iOS Lightroom from a MacBook. Shared link wouldn't populate the preview image despite four previous albums uploaded in recent days showing the preview link fine.
I fixed it by going into Lightroom cloud via the iPhone Lightroom app and changing the album cover from within that app.
To do that, when viewing your cloud album within Lightroom on your iPhone select the photo you want for the cover, on the photo view click the upper right three dot menu and select "Organize" then select the option to set that new image as your album cover.
For whatever reason that forced Adobe to reset the link preview cache with the new image and upon sharing the link in WhatsApp the preview image finally showed up.
Hope that helps someone else out there with the same issue!
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