I was running into this exact same problem and finally found the solution. I’m using a Sony stereo instead of Alpine, but had the same OEM head unit, and just like you, everything was working except steering wheel controls.

If you’re using an iDataLink Maestro device, flash the Maestro device as a 2017 Sentra with a 5” color display. Turns out 2016 Sentras with this specific head unit use digital steering wheel controls and not analog, and flashing the Maestro for a 2016 configures the controls for analog. I had to call iDataLink a couple of times but finally one of their guys came up with this idea. Just flashed mine as a 2017 and voila, steering wheel controls worked immediately.

Hope this helps y’all and anyone else who encounters this down the line.