I've figured it out.
It was a mismatch between the versions of Qt/Pyside6, not exactly sure which one.
Checking with
print("PySide6: v", PySide6.__version__)
I found out that the version that I installed in the virtual environment was newer (6.10.1) that the version available though apt (6.8.2.1).
Maybe it was a bit excessive, but I deleted the virtual environment and created a new one in which I installed the same version of pyside6 that is available through the distro package manager (6.8.2.1).
Adding this line enabled the selection of the breeze theme, the same as before.
app.addLibraryPath("/usr/lib/x86_64-linux-gnu/qt6/plugins")
The difference now is that when I use the "app.setStyle('Breeze')" command, it works.
Hope this is useful for someone.
