When you copy a Wiki page from a Wiki Page library to an Enterprise Wiki site, the Wiki page becomes empty at the destination. This is because your source and destination Wiki will not use the same content type since an Enterprise Wiki site keeps your Wiki pages in the default Pages library, and regular Wiki pages are kept in a Wiki page library.
It is possible to migrate a Wiki page to an Enterprise Wiki site with these steps:
How-to
Select Copy in the left menu.
Click on Copy content only.
Click Connect.
Select the library that contains the Wiki page you want to migrate within your site.
Click Next.
Click Connect.
Select the library where you want to migrate your Wiki page within your site.
Click Next.
Click Mappings.
Select the Content Types tab.
Map the content type Wiki Page to Enterprise Wiki Page.
Click the Properties tab.
Map Wiki Content with Page Content.
Click Save.
Click Options.
Select the Metadata tab.
Expand Page layout.
Select Set custom value and choose (Enterprise wiki Page) Basic Page.
Click the checkmark in the top right corner to save the changes.
Migrate your Wiki page.