When the pillager shoots using a multishot crossbow, the arrows all shoot in the exact same direction instead of spreading like when the player uses a multishot crossbow. Same glitch occurs with firework rockets as well.

To summon a pillager with a multishot crossbow, use this command:

summon minecraft:pillager ~ ~ ~ {HandItems:[{id:crossbow,Count:1b,tag:{Enchantments:[{id:multishot,lvl:1}]}}]}

Watch it fight and see that the pillager shoots concentrated clusters of arrows.