Tunnel ACK on Weinzierl Knx interface 731

During the recreation of an unload operation of a knx device (ETS > Device > Unload > Unload application) via QtKNX, I've encountered a problem with tunneling ACK.

This is the output I aim to recreate:

This is the output I currently have:

As you can see, the library does not ACK the DeviceDescriptorResponse, so my KNX device retries 3 times.

When browsing the source code, I can see that the ACK is send by the library, and it also shows up in the wireshark log: 

unload_via_ets5.pcapng

devicedescriptorread_qtknx.pcapng

But it does not show up in the ETS Bus Monitor (and is not transmitted on the KNX bus).

Can you help me with this problem? 

Thanks!

Jonathan.