Indeed the plugin is only supporting fullscreen cameras right now. The code is relying on screen coordinates, so if you change the viewport, then it won't work.
You can try to edit lines relying on screen like these :
Vector2 screenCenter = new Vector2(Screen.width * .5f, Screen.height * .5f);
float aspect = (float)Screen.width / (float)Screen.height;
Try to recalculate them to find the proper Camera screen center based on the viewport, and the new aspect ratio.
If it's not enough, then the problem might go deeper into the input system with LeanTouch that also uses Screen calculations:
HeightScreenDepth = new LeanScreenDepth(LeanScreenDepth.ConversionType.HeightIntercept, -5, groundHeight);
I will have a look on my side in the next days to see if I can support that. Keep me posted if you find a solution in the mean time!