Cancelling a bow shot and releasing Fire still causes an uncharged shot to go off while using the Full Auto setting on controller. I've also tested MnK and there is no issue.
Normally, this wouldn't be an issue but Leviathans Breath has limited ammo.
The safest work around is to swap weapons, just turn off the full auto setting or land your shots (gasp).
You can also prevent a shot being wasted is to cancel the draw and release fire within a few frames of each command.
I discovered this when using the new exotic bow and it's been pestering me ever since it's not that big of a hassle to work around, but it's also ... [i]there[/i].
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If I had to guess it's because triggers take longer to let go if that makes sense because the moment you stop pushing on the mouse it lets go but the trigger will keep going till it's 100% let go and fully pushed out again.