Dispel Grenade damage

When throwing a Dispel Grenade, swapping to a regular grenade or throwing knives before it lands will add the damage of the other off-hand ranged weapon to the Dispel Grenade, giving it both dispel and damage.

I couldn't reproduce using the spell version of Dispel and switching to grenade. It may be possible that the spell also has a similar reaction under other circumstances, which I've seen in 0.3 and previous versions, but I have not been able to figure out if it was random or triggered because of how rare it was.