This article describes and issue with saving Metafields when working with OrderAddress.  When trying to save the payment billing address as follows:

IOrderAddress address = GetDefaultBillingAddress();
address.Id = Constants.Order.BillingAddressName;
address.Properties["IsValidated"] = true;
address.Properties["UserChoosetoOverrideValidation"] = true;
payment.BillingAddress = address;

ServiceLocator.Current.GetInstance().Save(cart); 

All properties are saved properly except the MetaFields. They are all NULL.

This issue was first reported in the post here in EPIServer.Commerce 10.4.  The issue was resolved in EPiServer.Commerce 10.4.3 which can be found here.  The recommendation is to update the nuget package to that version or later version.