Outlook 2016 desktop Add-In does not retain the company domain

Occasionally, I get reports from end-users that the  Add-In (not Plug-In) for Outlook 2016 desktop is not keeping them logged in via SSO.  When the sign back in via SSO, the company domain is not filled-in in the domain string.  If anyone knows the issue and resolution, it will certainly help.  The Add-in is deployed via the Exchange server if the end-user is a member of the AD group. 

Thank you!