Despite not requiring login, users have to log in every time they click a proof link

Greetings! 

Trying to figure out how to avoid having document proof recipients need to log in every time they click the proof link from the proof email.  Have validated that “require login” is not checked on both the individual proofs and the admin section.  These are all people who have had the proofs assigned to them as part of the initial proof creation process and not shared later.  Using the basic workflow for proof creation as well.  Oddly, as the person who is creating the proofs, I never have to login when clicking the link in the proof email, but anyone else the proof is assigned to has to sign in every time they click the link, regardless of whether or not they were already logged into Workfront.  We are using SSO. 

Thx in advance!