SAND doesn’t take any midi input from midi output of standalone midi controller app like Velocity Keyboard and Midi Poly Grid. If the apps support AUv3, they work perfectly after being inserted as midi fx layer under SAND track.
I am not sure if that’s a bug as other host like AUM and Groove Rider 2 can take both added FX layer and standalone as midi controller. I checked and seems it should be supported by Core Midi.
If standalone app is allowed, users have more choice of midi input interface and also doesn’t have to host the same controller on all tracks to leave the layer for other plugins.