Cross-domain autologin on Domain Mapping doesn't work

It seems like the cross-domain autologin on Domain Mapping doesn’t work like it used to. In the past, a user could log in through my main site, and it would redirect them to their own admin area/dashboard.

They would automatically be logged in to their own site. But now that doesn’t work. User can log in on my main site and it redirects them to their own site’s login screen, but they are not logged in and must log in again.