The bug

Unlike other non-living entities (items, boats, etc.), armor stands are destroyed by explosions created by fireworks. They will drop any armor and held items, but not the armor stand itself. This does not affect invisible or marker armor stands.

To reproduce

1. Give yourself a crossbow already loaded with a firework rocket by using the command provided below.

   /give @s minecraft:crossbow{Charged:1b,ChargedProjectiles:[{id:"minecraft:firework_rocket",Count:1b,tag:{Fireworks:{Explosions:[{Flicker:1,Trail:1,Type:1}]}}}]}

2. Place down an armor stand and shoot the firework rocket at it.
3. Take note as to whether or not the armor stand is destroyed by the firework explosion.

Code analysis

Armor stands are the only non-mob/player entity that's classified as LivingEntity, and firework rockets damage only LivingEntities within a certain radius, as opposed to all entities (like normal explosions do). Armor stands break without dropping themselves if damaged by anything classified as explosion damage, which includes fireworks.