This seems to be a long-standing issue. When the body rotation (yaw) of a monster entity (and possibly other entities) is set, it does not update in-game. Several forum suggestions include teleporting the entity after setYaw(), setting yaw before spawning the entity, and using NMS code, all with limited or no results. Same result with /summon command.

Sources:
(Best example) http://www.spigotmc.org/threads/direction-yaw-and-pitch-not-working.50972/
http://www.spigotmc.org/threads/set-body-rotation-of-a-livingentity.46377/
https://bukkit.org/threads/setting-yaw-doesnt-do-anything.71451/
https://bukkit.org/threads/entitys-yaw.316961/