Ubuntu 16.04 / ASUS 8800GT
Windows 10 / Titan X Maxwell
Short description of error
Using an added Cube in combination with Particles > Volume > Grid results in missing particles.
Strangely, there is no issue when you use the original Cube "Cube", or a duplicate (Shift + d) of the original Cube. ??
Not sure if this will help, but I found that if you check 'Invert Grid' under 'Emission' for the added Cube, the missing particles appear: