@Andrew Sader, Thanks for posting in Q&A. Based on my previous testing in my lab. I find when I choose one OU with the devices which I want to do Hybrid Azure AD join to enable Password Synchronization, only the devices in this OU will do Hybrid Azure AD join, others will not.
You can reconfigure the "Custom synchronization options" in Azure AD connect to see if it can meet your requirement.
Hope the above information can help.
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