I have dug deeper and found out that the behaviour you are encountering is actually a bug when having a redirect page under a group page ( such as the content group page ). We have a PITS
item that was previously created.
As a workaround I can suggest you to put the redirect page outside the group page, or create a standard Sitefinity page, create a codebehind file for it and perform a Response.Redirect action in the code behind to the external page.
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