I'm having this issue when working with the new Shadow Caster 2D in Unity.
https://www.youtube.com/watch?v=tvUNsYCVeIM&feature=youtu.be
A quick fix is to increase the outer radius and raise the falloff intensity to max to compensate.
Dev told of another temporarely fix, which is to add a vertex to your shadow caster in the middle of each of your sides. I haven't tried it myself yet, as I've found solution 1 sufficient for testing purposes, but i'll probably try it out as some point.
Ultimately its a bug that they will fix it in time.
For futher questions regarding 2d Lights/Shadows I recommend the thread on Unity: link