Whenever you charge a crossbow, it saves the list of itemstacks that need to be shot out of it at time of loading. This means that if there is a multishot enchantment, it clones the itemstack and stores it three times and saves it to NBT. 

The multishot enchantment is not checked beyond the point of loading the crossbow. Instead it pulls the NBT list of items whenever the crossbow goes to be unloaded and iterates through the items that need to be converted into projectiles.

This means that charging a crossbow with multishot and then removing the enchantment from a grindstone will still result in the crossbow firing three projectiles and damaging the crossbow the incorrect amount of times. (See MC-215540)