Custom Sync Filter Issue

How does the custom sync filter behave for Account & Contact Object synchronization (Person Account)? We observed that when the Custom sync filter (Account.Marketo_Sync_Required = false ) and the ( Contact.Marketo_Sync_Required = true ), updates made to the Account object are still synced to the customer in Marketo. However, updates made to the Contact record aren't synced to Marketo. On the other hand, when the ( Account.Marketo_Sync_Required = false ) no updates made to the record are synced to Marketo.

Does anyone have any experience on working with Custom Sync Filter ? We want to understand how this behavior works between Marketo and Salesforce.