When adding an external link in edit mode, if the link exists in the system it will be converted to a permanent link.
This is intended functionality for the system. External links are mapped to permanent URLs if the host part can be mapped using the site settings. The reason is both to support multi-site configurations but also when editors copy-paste URLs from the site and insert in edit mode.
URL's where the host part cannot be mapped as per the site settings will not be converted to permanent URLs.