This bug appeared from snapshot 18w46a.

The bug

After 18w46a, mobs armed with a crossbow do not attack the player for a while, even if their crossbow is loaded. Does not apply to mobs armed with a bow, so I considered it a bug.

This is very noticeable if the mob's crossbow has a level 3 "Quark charge" spell.

How to reproduce

1,Summon pillager with this command:

/summon minecraft:pillager ~ ~1 ~ {CustomName:"\"Pillager\"",CustomNameVisible:1,Health:9999,Attributes:[{Name:"generic.max_health",Base:9999}],HandItems:[{id:"minecraft:crossbow",tag:{Enchantments:[{id:quick_charge,lvl:3}]},Count:1},{}]}
You will notice that when the pillager crossbow is loaded, it does not fire immediately, which makes the "Quark charge" enchantments completely useless to them. Also works with piglins.