Been searching for a couple of days now and haven't really found anything related to this issue. URLs in WP post/page content linking to other internal posts/pages are all absolute and when using WP as a headless CMS the domain needs to be replaced.
Gatsby for example has a plugin with a built in feature that does that automatically: https://www.gatsbyjs.com/plugins/gatsby-source-wordpress/
// Search and Replace Urls across WordPress content.
searchAndReplaceContentUrls: {
sourceUrl: "https://source-url.com",
replacementUrl: "https://replacement-url.com",
Is there anything similar available in/for next.js or do you have to do it "manually"?