Network admin redirect and authentication question

Say I have a simple WP multisite install (if there is such a thing ?? consisting of 2 domains.

DomainA.com is my master domain, and DomainB.com the other domain. I’m using Domain Mapper.

I’m logged into wp-admin as a network admin user on DomainB.com.

I click Howdy Admin -> Network Admin and get redirected to wp-login on DomainA.com … and have to enter my un/pw again.

I could’ve sworn that was working before but now it isn’t and I can’t recall if I’ve broken something or if it is due to cookies not working across domains…