AE doesn’t do raytracing – you have to duplicate your layer and turn it up-side down to fake the reflection.
To have the reflection falloff you can simply add a mask with feather or better create a new solid on top of your reflection layer with a black&white ramp applied. Then set your reflection layer to use the ramp layer as a luma matte. Add some blur for greater realism.
check out
https://www.reidroberts.com/aeprojects/
“Faux Raytraced Reflections”
(In order to save the project file right-click, save as and change the .htm to .aep)
Good luck,
Andy