This is actually using the existing explosion which occasionally suffered from the same bug. It's an issue with the way the textures are called, needed to be looked into anyway. - Durandal.
I tried it in SP and the explosion looked fine for me, so I don't know what caused this issue for you. Either way, the explosion effect is getting changed in an upcoming hotfix. About the projectile itself, I decided to use that one so that we have 3 different effects for the 3 existing mortars. ~ChillerMiller