Why does my DPD tracking code automatically switch to DHL?

[dorst]

When processing an order, we manually put in the tracking code of our carrier, which is DPD. 

After confirming the code, this then automatically switches to DHL ecommerce with the same tracking code and the customer gets a shipping confirmation email. When they click the code in the email, they are directed to the DHLR website with a non-functioning code. 

When they are not home at the time of the delivery, they also don't get a notifications of their shipment being delivered somewhere else for pick-up.