Hi @Kevin Talbot,
Based on your query, I understand you have an issue with cloud sync SSO.
I would like to provide you the document to enable SSO with Cloud Sync: Using single sign-on with cloud sync. SSO configuration will be done for the entire forest which might help you in having SSO for the all the users. Also please go through this document for troubleshooting the Seamless SSO: Troubleshoot Microsoft Entra seamless single sign-on.
I hope this information is helpful. Please feel free to reach out if you have any further questions.