@meynti hi there. I briefly came across this problem but figured it out quite quickly.
This control pad has 4 different methods of how the controller will work.
Nintendo switch
Dinput
Xinput
Mac OS
Same as you. Mine was setup to switch mode. Works with the pie. But not steam link.
Dinput also doesn’t work with steam link.
Xinput did work perfectly.
Haven’t tried Mac OS mode but would suspect it wouldn’t work.
Here’s how to sort it.
Get a keyboard ready or another controller that is setup to work on your pie. . You’ll need it soon.
First of. Go to RetroPie Bluetooth settings. Remove your controller from the list.
Now. Turn off your controller. Hold start for 10 seconds.
Hold X and press start to turn the controller on. The X will turn it on in Xinput mode. Two lights should be flashing below the sticks.
Now add your controller again on the pie.
Once done you can quit the Bluetooth settings. You’ll need to go to the bit to configure inputs. If you have another controller that already works on the pie then the start button will bring up this menu you need. From here reconfigure your 8bitdo controller buttons.
Once you’ve done that the steam link will now see your controller. It will say it’s a Xbox one controller but it will work fine. It did for me.
Hope this helps.