The documentation of QLowEnergyService::characteristicWritten states that the newValue parameter may be different from the corresponding write request.
However, from my (superficial) view of the bluez implementation (qlowenergycontroller_bluez.cpp) that doesn't seem to be possible. Also I wasn't able to find anything about that in the bluetooth spec. At least a Write Response (Spec v4.2, Vol 3, Part F, 184.108.40.206) does not echo the written value back.
Is the documentation wrong here or am I missing something?