According to following URLs, Custom Field is not able to sync.
However it sometimes synced when we actually tested.

Title : What fields can be synchronized between CRM and CRM for Outlook?
URL : https://technet.microsoft.com/en-us/library/dn832089.aspx

Title : View the fields that are synchronized between CRM and Outlook
URL : https://www.microsoft.com/en-us/dynamics/crm-customer-center/view-the-fields-that-are-synchronized-between-crm-and-outlook.aspx

Seems like its triggered by "timing of click on the [Synchronize] button" and/or modifying syncable Field and Custom Field.
But we are not sure..,

Microsoft should provide workarounds.
This functionality needs to be stable for better product performance.

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