How would you have expected the shadows to be cast with the light behind the target?

This is how shadows work in real life as well. This is not a problem with Aximmetry and it’s not a problem with UE4 either. 

 yes, I know that

I mean the distance between the shadow and the target

Ah, ok.

You just need to align the feet of the Billboard actor to the floor properly. 

Keep in mind that shadows cast by a billboard isn’t cast by a true volumetric object, since the Billboard is only a 2D plane. This will cause the shadow to look strange  or misaligned in some cases.

I aligned the target with the ground, but when the camera rotated, the target appears higher than the ground

Hi Huseen,

You will never get precise alignment with the floor since the billboard technique (placing a rotating 2D plane into the 3D scene) is a cheating.
In order to resolve your issue I recommend using the AO feature of the billboard (fake shadow spot).  With proper settings it can hide the gap between the feet and the shadow.