Pillagers don't have any arms if they aren't carrying a crossbow

The bug

Pillagers don't have any arms if they aren't carrying a crossbow.

To reproduce

1. Spawn a pillager
2. Spectate it
3. Run

   /replaceitem entity @e[type=pillager,distance=..1] weapon.mainhand minecraft:air

   → It no longer has any arms! (Well, it no longer has a crossbow, and it has no actual arms as well.)