Cross domain autologin doesn't work

We created a brand new dev domain the other day and got our multisite setup and working. On that dev domain, as a Super Admin, we are able to move between subsite dashboards and are never asked to log in.

We’ve migrated that dev multisite to our live domain now, and until yesterday (I think) things were working fine. We noticed today though, that logged in as any Super Admin account, we are unable to visit subsite dashboards, the issue happens only with subsites which have mapped domains. We get prompted to log in. If we enter our Super Admin credentials, it works, but I don’t know why we’re being prompted to login in the first place.

We’ve tried enabling “Cross-domain autologin”, but it doesn’t work.

We’ve tried default .htaccess file and plugin conflict test, but it doesn’t give any results.