Rumble overheat and E.

So I'm loving Rumbles ammo system on his Electro-Harpoon, but I'm a tad confused on how it works in conjunction with his Overheat mode. I've noticed that occasionally I can cast Electro-Harpoon while in overheat mode, and other times I can't. At times I am able to cast it within a second or so of overheating, other times I cannot. Oddly enough if I'm more than 2 seconds into Overheat mode I absolutely cannot cast Electro-Harpoon no matter what the circumstance. I have no idea if this is a bug or a feature but it feels horribly inconsistent to me. I would love someone to explain this.