Remaining URL field is blank after saving external link to CMS page with action route

Description

Steps to reproduce:

1. Add a custom action route method (e.g., MyAction) into the PageControllerBase class so we can get action via URL: <Page1's URL>/MyAction
2. Add a link with external link, such as <Page1's URL>/MyAction, to a page's property.
3. Open the Edit Link dialog again.

Expected:
"MyAction" is shown in the Remaining URL field, and Page1 is shown in the Page field.

Actual:
Remaining URL field is blank.