Facebook has made a lot many changes in its API where one of the change is you cannot customize your link preview - image, title, description effective from July 17th, 2017. This is part of Facebook's effort to reduce fake news and spread of misinformation on their platform.
The changes to the link preview apply all third party apps like SocialPilot.
Here's Facebook official documentation on changes in the link preview: "We're working to find other solutions that allow publishers to share customized content on our platform, and we will have more to share in the coming weeks. As we make these changes, content creators maintain the ability to control how their links appear on Facebook using Open Graph meta tags."
By removing the ability to alter link metadata (i.e. headline, description, image) from all link sharing entry points on Facebook, they are eliminating a channel that has been abused to post false news.
Facebook also rolled out a new feature for certain publishers, called "Link Ownership." It's a new tab under Publishing tools and available only for the media pages
Facebook's official comment on link preview editing and Link Ownership "We introduced a way for publishers to indicate link ownership and continue editing how their own links appear on Facebook. We know media Pages use this feature daily to customize and post high volumes of links they own. Pages eligible for this tool will see a new section in their Page's Publishing Tools called Link Ownership" On September 12, 2017, we will complete the feature removal such that publishers' Page(s) will only be allowed to edit links from website domains they have authorized with the link ownership tool.
If you would like to make it more clear, kindly consider one of the following.
The users who have connected Facebook Profile, Page or Group by default setting with SocialPilot they will get the same post shared on Facebook which they have scheduled by Custom Preview in SocialPilot.
The users who have connected Facebook Profile, Page, and Group by using Custom Facebook Branding, The posts they will schedule through 3rd party tool like SocialPilot won't be shared as custom preview due to Facebook API limitation.