The bug
Based on MC-87752 and the fact that armor stands hold shields like mobs and players armor stands should have arms to hold a shield.
Despite that dispensers are able to equip shields for armor stands without arms.
MC-112533
Dispenser can equip shield for armor stand without arms
Resolved
Fixed
4
Confirmed
armor_stand, arms, dispenser, shield
Minecraft 1.11.2, Minecraft 1.12.2, Minecraft 1.13-pre1
Minecraft 18w32a
Retrieved
This is also a problem in 1.12.2. The armour stand must be destroyed to remove the shield.