Relates to MC-152071
The Bug:
Summoning a crossbow that is loaded with a projectile that doesn't have an item form, results in a crossbow that is loaded with "air" and shoots a blank shot.
To reproduce
/give @p minecraft:crossbow{Charged:1b,ChargedProjectiles:[{id:"minecraft:small_fireball",Count:1b}]}
or
/give @p minecraft:crossbow{Charged:1b,ChargedProjectiles:[{id:"asdf",Count:1b}]}
[media]This is inconsistent with how other items that are not viable for charging in a crossbow are handled. Normally, when you summon a crossbow with an item that cannot be shot by a crossbow, it will be in the tooltip, but shoot an arrow.
[media]
Air is simply the default item; and yes, air items exist, but instantly delete themselves.
The tooltip appears whenever the "Charged" NBT is set, that's by design.