Because of using Invert Curve for Falloff Density this is applied also to your Exclude Area calculation. Modifying of this algorithm could lead to another undesired effects, but we'll see if we can do some tweaks in this part for future releases.
In this case, we recommend you to use the spline already "prepared". This could be done easily with Boolean Subtraction operation. Please check attached screenshots.
Hope this helps you a bit and if there will be anything more we can help you with, please let us know. Have a nice day.