Thanks for your inquiry.
Yes, WORDSNET-16579 issue was closed as “Not a bug”. The latest version of Aspose.Words for .NET 18.7 generates the correct output for fields that have format switches.
There was an issue in older version of Aspose.Words 17.3 and was fixed later.
You have implemented IFieldMergingCallback interface. In this case, The field format is not applied. This is expected behavior of Aspose.Words.
In your case, you need to list down all your cases that your customers are using. Please format the date and number values using .NET API in IFieldMergingCallback.FieldMerging.