The Apple adapter is unfortunately not compatible with DisplayPort or Mini DisplayPort monitor signals, despite the cables and ports having the same shape.
It can only be used to bridge Thunderbolt signals, not DisplayPort.
This is confirmed in the product notes:
Note: This adapter does not support DisplayPort displays like the Apple LED Cinema Display or third-party DisplayPort and Mini DisplayPort displays. For more information, visit the Apple Support page for the Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.
This adapter is not compatible with displays that use DisplayPort or Mini DisplayPort, such as the Apple LED Cinema Display. Thunderbolt and Thunderbolt 2 are not the same as Mini DisplayPort, though the ports have the same shape.
This adapter does work for the Apple Thunderbolt Display, but not for the Apple Cinema Display (DVI or Dual-Link DVI) or Apple LED Cinema Display (Mini DisplayPort)
Apple does not manufacture a first-party Mini DisplayPort to USB-3 adapter.