[Defender Pro] Issue with mask login area

I am using the Mask Login Area feature of Defender Pro.

On some of my sites, when I go to my new login URL, I am presented with the login screen, but then when I successfully login, it redirects me to the website home page, rather than the admin.

If I then re-visit my admin URL, I am logged in – however, this isn't a great user experience for my clients.

What could be going wrong?