I have finally found the cause of this annoying occurrence. If you switch the direction of the guard right after you've inputted the heavy attack it will become a ranged heavy. This ranged heavy comes out as soon as it has the minimal charge to reach lvl 1 (You have lvl 0 which you can cancel to let your chain spin once, lvl 1 that has short range, lvl 2 that has max range). If you repeatedly input the light attack with this glitch you will release the heavy as soon as it reaches charge lvl 2.

*edit I play on Pc if it matters, I don't know if this is an issue on other consoles.