JiraSOAPService.progressWorkflowAction delete custom field values when they are not part of the RemoteFieldValue argument

When using JiraSOAPService.progressWorkflowAction all Custom Field Values are deleted if they are not part of the RemoteFieldValue argument

I am not sure if this is a bug or a feature ?

but it would be nice if the values in the custom fields were only changed if they were included in the RemoteFieldValue argument