Hi Paula. You don't need to be concerned that it shows the offer as Target Classic. This is working as designed . Since you are using the API , to create the offer the UI displays it as such, meaning its not created in the target standard UI. Your offers will not stop working because of that.
If you would like to change the USER/name on the Last Modified field, so we can know which developer actually made that change with the API then we recommend that you submit this as a feature request through