Preserve custom field values in anonymous leads that are merged with known leads whose fields are empty

When an anonymous leads is crated during a visit to our website, we set values in ithe newly created record's 'Initial Online Source' field and other fields that tell us the source of their initial visit to our site.  Later, if a known record for the same lead is created by Sales, we don't set the Initial Online Source field and leave it blank.  Then when we send an email to the lead and the lead clicks the tracked link, Marketo merges the anonymous lead  record with the known lead record.  However, the resulting merged record's Initial Online Source field is empty.

This is unexpected as the 'Merging Leads: Behind the Scenes' help article (community.marketo.com/MarketoArticle?id=kA050000000KyszCAC) states that, "When merging a lead with an empty field with another who has a value for that field, the merged lead will have a value in that field."

Paradoxically, Marketo does clear set a standard field's value to empty when automatically merging an anonymous record with a field value and a known record that has no field value.

My suggestion is that when Marketo authomatically merges an anonymous and a known lead record, any field with a value should be preserved if the other recrod's field is empty.