This issue is not because of smoothing/normals.
It is because you are comping the elements in PS in 16bits, which results in too coarse color values, which will inevitably produce banding in dark areas. You have to use 32 bits in PS.
When you save out elements, you need to make sure to use 16 (or 32) bits in an HDR floating point format - for example EXR.
HDR formats can use 16 or 32 bits, both will work for you, just use EXR. HDR formats save values as floating point data which is needed for increased color value accuracy.
Note that this is *not* the same as using 16 bits in LDR formats such as TIF/PNG. These LDR formats use integer values which have limited accuracy. This is what you're seeing - either because you saved elements to a integer LDR format or because you're working in 16 bits mode in PS (or both).
In Photoshop/After Effects, you have to work in 32 bits (it's always linear) mode when comping elements.
