I assume it the authentication redirect, as accessing it locally does the redirect e.g.
https://admin-dev.testsite.com/ ->
(302) https://admin-dev.testsite.com/admin ->
(302) https://admin-dev.testsite.com/admin/ ->
(302) https://admin-dev.testsite.com/admin/CMSAdministration.aspx ->
https://login.microsoftonline.com...
Test Site
https://admin.testsite.com/ ->
(302) https://admin-dev.testsite.azurewebsites.net/admin ->