If you migrate a page that already exists at the destination with Copy content only, that page is overwritten with the source's page.
When SharePoint performs a validation at the destination afterwards, it will see that page as deleted, and the link to the page in your quick navigation will get removed or end up broken.
To fix your quick navigation, you can manually re-create or correct the link after your migration.
You can also use Copy structure and content to migrate your entire pages library instead. This way, the app will recreate all the pages and navigation at the destination.