SEGA Genesis/MegaDrive Mini USB controller on macOS

[pkZip]

I recently bought the "SEGA Mega Drive Mini" in the UK (known as the Genesis in the USA), which comes with two USB gamepads. They are relatively simple, just a D-pad and four buttons (A,B,C, and start) but that is plenty for many games on Steam.

I have plugged the device into a Linux computer, and via SteamLink could use all the buttons. i.e. the Linux drivers seemed to work just fine. The UDB ID was 0ca3:0025, Sega Corp.

I want to use this on macOS, but it was not plug-and-play with Steam. I opened Steam preferences, controller settings, and it lists a "3B Controller" under Detected Controls. If 3B means three main buttons, so far so good. It lets me assign A, B, C, and start (reported as buttons 2, 1, 5 and 9). However, if I try to configure it the D-pad shows "Axis 3" for left or right, and "Axis 4" for up or down (can't set both). Confusingly the page only has "Cancel" and "Clear and exit" buttons, so I can't even save a partial mapping.

Is this a Steam problem, or just due to missing appropriate USB drivers on the macOS?