When trying the lowenergyscanner example with 5.9.3, the application logging reports:
qt.bluetooth: Dummy backend running. Qt Bluetooth module is non-functional.
When trying trying the same with 5.9.2, everything is ok and works. Also when looking at the symbols in 5.9.3/gcc_64/lib/libQt5Bluetooth.so.5.9.3, nothing is found containing bluez, while doing the same on 5.9.2 returns lots of symbols referencing bluez.
It looks like the 5.9.3 build is not properly built for Bluez and thus only contains a dummy backend.