Can't access admin through mapped domain

Hey all, having a weird issue here.

I have a main domain of xyz.com.

I have about 70 sites all set up with an individual mapped domain so that they can be access through abc.com, etc.

The issue I am having is that all the sites work fine. However, when I try and access the sites wp-admin through abc.com/wp-admin, it doesn’t work. It keeps redirecting me back to the abc.com/wp-admin page with no errors. No invalid password or anything.

When I access it through xyz.com/site-path/wp-admin (main url for the sites), it works fine. I’d like to remedy this due to the fact I don’t want all of my other site users accessing their individual sites through xyz.com/site/path/wp-admin and instead going to their mapped domain of abc.com/wp-admin.

I feel like this may be a very common problem, but I haven’t been able to find anything. I’m sure it’s a setting somewhere or something that I am overlooking, but I haven’t been able to resolve it yet.

Any one have any experience with this they’d like to share?