[Defender] Defender Login Mask with Mapped Domains

I just implemented the Defender feature to Mask Login and I am no longer able to login to the sites with mapped domains if I have selected the mapped domain as primary. When I try to use the old login slug wp-admin I get an error page that says, "This feature has been disabled" and when I try the new slug I get an error 404 page.

Should I assume that the Login Mask is incompatible with mapped domains or is this a bug?