When an elite mortar fireball hits the slow time shell it falls vertically to the ground - I'm guessing its some sort of coding bug dealing with the vertical arc of the mortar. Purely horizontal missiles seem to be fine.
An effect of this is that if a mortar elite gets inside a slow time shell then all its mortar rains down inside the shell making using the slow time shell pretty dangerous for such mobs
The same effect can be seen with any missile that has a vertical component such as lacuni prowler grenades
I really like the spell graphic and use - it would be great to fix it