Qt + VB-CABLE : 8 channels on macOS / 2 channels only on Windows

I'm using VB-CABLE to capture audio from another application : Reaper --> VB-CABLE --> my Qt application

The same code runs under macOS and Windows (Linux too, but that's another story, there's not easy equivalent to VB-CABLE for this OS).

Under macOS, no problem, Reaper outputs 8 channels, VB-CABLE gets them, and give them all 8 to my application.

Under Windows, I can get only 2 channels from VB-CABLE.
When I list all the audio inputs, using QMediaDevices::audioInputs(), I find "CABLE Output (VB-Audio Virtual Cable)" which has only 2 maximumChannelCount(). It should return 8.

Am I doing something wrong ? Is it some kind of parameter somewhere ?
Or may be Qt is using a limited Windows API to access audio devices ?
I read that there are many of them (more or less capabilities, optimized latencies, etc). If this is the case, is there an existing solution ? Or would it mean refactoring the Windows version of Qt to switch to an API that would allow more than 2 channels ?