Noticed it few hours ago.
While doing PvP match Invocation can absolutely random reset its cd. After noticing this i made a few matches and twice i had it reseted after killing blow on target with Anvil of Doom(GF skill), once after my death.
Not much but just if ur doing pvp for leveling just watch for this and you will get some extra diamonds and experiance.
PS sory for my poor english.