Just tested this and I am unable to reproduce.
You are using the most recent build from MaNGOS ONE Server on git?
Could you provide some more information for us, such as. are you casting the spell with the .cast command, do you have any talents that alter the spell, such as Improved Shadow Word: Pain. What auras do you have on your character, Do you have an armor set on that alters the spells ability?
Any information that would better lead to reproducing would be great. Also, does this happen every time, or is it random?