When I use THREE.Sprite with mapbox the sprite is always horizontal and the sizeAttenuation property is not working for scale.
I understand that sprite position depends on camera projection which depends on the mapbox matrix.
I have no experience with Mapbox. Most likely it has its own camera, outside your Three.js world. The sprite works with the Three.js camera, so my guess is that you should find a way to get the Mapbox camera parameters to your code, so that you set the Three.js camera to be the same as the Mapbox camera. Then the sprite should work as intended.