Hello @Rob
Thank you for reaching out. As per your explanation I assume that SSO for OneDrive Sync Client still does not work, and user still has to login to OneDrive Client one time in before SSO actually kicks and works for subsequent connections of user logging on.
If that is the case, you can try to detail mentioned on the following document: Silently sign in users to the OneDrive sync app with their Windows credentials
I hope this helps.
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