Macbook FSSO weird issue

having issues with some macbooks not getting authenticated to FSSO (FSSO client runs on our DCs)

I start by opening chrome on the macbook, chrome goes to google home page initially, when I try to go to another page, say I select msn.com, I get error: 

Your connection is not private. Net::err_cert_autority_Invalid

but - if instead I try a different webiste, say CNN,  I then will get the authentication prompt for my AD credentials, and have internet access.

I am able to replicate this each time by de-authenticating my login on the fortinet and trying again.

each time I select MSN it fails with the cert error, each time I select CNN I get signon prompt.

why does it matter what website you attempt to access to get the login prompt?