Usually there is a note on the bill about what has been done and you see repaired/exchanged parts listed. If they haven't found any problem, there was no repair and no exchange of parts.

This would mean that you have to check the mixer again and make absolutely sure that the problem is NOT on your side. If you are absolutely sure and if you can describe how to find the problem, contact your service again and insist on a "second round". It might be helpful to open a ticket first, because the guys in the service department have approach to any relevant information of your (registered) mixer.

https://support.presonus.com/hc/en-us/requests/new