I don’t know if I said this before, but a page link field located in one place is not aware of changes to the slug for a page. The link field stores the current url when it is updated.
This same thing will happen when you add a link in WP to the content area. If you create a link in the WYSIWYG using the WP add link filter you will experience the same issue of you then change the slug of the page you are linking to.
ACF uses the built in WP link field and the ACF field is just a wrapper for WP functionality that already exists. In order to have the field and links updated when a slug is changed would require searching the content and all custom fields for every post to find the old link and replace it with a new link.
The solution I employ is to install a redirect plugin, there are several available. I work at an SEO company and everyone here knows that when a URL on the site is altered that 301 redirect must be added. This is because in order to get search engines to index the new page instead of the old page that these redirects need to be put in place. A side effect of using a redirect tool is that these links that get broken by changing a slug do not result in a 404 page. My company also employs tool for our clients that scan internal links to find 404s and redirected pages so that these can be corrected later.
Automating the updating of all links pointing to a page when a page slug is changed would be extremely difficult, if not impossible to achieve.
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