When shooting a crossbow, firing a firework rocket takes three times the durability. This is independent of the multishot enchant (which intentionally uses 3 durability), and when both are used it takes a total of 9 durability. This isn't listed anywhere on the snapshot announcements or wiki (unlike the multishot enchant).
MC-150640
Shooting a firework rocket from a crossbow takes 3x durability
Resolved
Works As Intended
2
Can confirm. This is probably intended though.