When importing Currency Codes with a Purchase Journal entry sometimes the Currency Code will be blank in Business Central, but no error message is returned.
The reason for this is because if a Vendor is assigned as the Balance Account the Currency Code comes from the Vendor card and can not be set by the Web Services.
In the below Screenshot you can see two lines were imported. The first line has a G/L Account as the balance account and it imported the Currency Code correctly, but the second line uses a Vendor as the balance account, and the Currency Code was not populated.
The work around is to leave the Currency Code blank when balancing with a Vendor.