Quick Charge Enchantments Makes Crossbow Unusable Pass Level 5

This is not a bug. Quick Charge 5 makes the crossbow take 0 seconds to charge, so anything past that would require a negative time. Since you start past that time without triggering it, you have to overflow the charge time value, which on its own takes a REALLY long time. And in the end, that's only halfway there. Your charge time goes to the other end of the spectrum, which is where the negative numbers are, making this possible. Eventually you will get to your crossbow's charge time and it will charge. But that crossbow must be really special for you to actually charge it, as the process takes 69 years.
This probably won't be removed due to the fact that you have to have commands to get past quick charge 4, and that nothing's actually going wrong. It's just how the game handles negative values. You cannot get a crossbow that charges faster than quick charge 5, because quick charge 5 charges the arrow/firework rocket instantly.