Creating an external link with the same hostname: when looking at link again, it turns into internal link

Description

When creating an external link with the same hostname, it turns into an internal link when looking at the link again.

Steps to reproduce:
1. In edit mode on a xhtmlstring property, in the Tinymce editor, create an external link to a page on the same site. For example, if the site is test.local, you add an external link to test.local/alloy-plan/#1234.
2. Publish the page.
3. Note that the external link is rendered correctly in view mode.
4. Go back to edit mode.
5. Edit the link.

Expected:
The Page radio button should be selected, and the query string parameters should be configurable in the "Remaining url" field.

Actual:
Link is turned into an internal link, where you can't see the #1234 part.