Upsert Salesforce Records using Reference Field

I have a flow setup where one of the destination field mappings is supposed to reference an external lookup field on the parent object. In this case I have a Child Object who's parent is the Account. On the Account I have a text field used for external Ids called Fieldin_Id__c. So the mapping is setup like this:

However, when I run the flow it does not set this filed using the lookup value. When using the same mapping in DataLoader it works just fine though. Has anyone been able to get this to work and if so how?