Data integration is not respecting the key fields

[Mglat]

I created an data integration project to import a simple Excel sheet into a custom entity. The data is imported in a CDS (D365 v9) environment. If I run the project several times, every time a new record is created. It is not looking for an existing record. I created an extra key and selected this on (just above the field mappings in 2nd step). 

Am I missing or misinterpreting something, or did I encounter a bug. 

For the record, I want to be able to do upsert scenario's on non-primairy field(s)