HDL - Element Entry Create record thinks it's updating an old one

Summary

even though I've supplied a unique SourceSystemId, HDL is trying to update an old, already end dated record instead

Content

Hello,

I'm a bit stuck at the moment.  I have a .dat file whose intention is to create an Element Entry.  I am using the SourceSystemOwner and SourceSystemId to generate the new user key for the record.  My understanding is that I can use anything as my SourceSystemId, and then, under the covers, fusion updates all the necessary tables with what it needs in order to insert the record.  To keep this SourceSystemId unique, I'm using a little formula : "ElementName_AssignmentNumber_EffectiveStartDate"