From your description I take it that the configured deeplink for external users the same as the configured deeplink for the internal user.
The main difference is that the internal user is already in the domain of the company. So probably you best contact the DSM it department and ask them what triggers this reroute.
Do you have SSO configured? Because I think that the combination of internal SSO users that uses the deeplink and external users using deeplink but with the standard login page might be impossible. It could be solved by creating accounts at the IdP. But I can imagine they do not want that.
Did you see this part of the documentation?
LoginLocation - If a user session is required this constant defines the loginpage where the user is supposed to enter the login credentials. This property is useful in single-sign-on environments. If empty, the default Mendix built-in login page is used. If not empty, it is assumed that after login, the user will be redirected to the deeplink again. For this reason the provided url is appended with the original deeplink. For example: 'https://mxid.mendix.com/login?a=MyApp&f=true&cont=' or '../sso/login?f=true&cont='