Your default Microsoft Dynamics prefix for custom fields must be new in order for Marketo proprietary fields to sync correctly. Here’s how to change the default prefix.
Go to Settings and select Customizations.
Select the default publisher from the list.
Change the prefix to new. Click Save and Close.
Go to Settings > Solutions to publish the customizations.
Click Publish All Customizations.
Now, create your custom fields. After you complete them, revert the prefix to the original one.