Starting from version "git-Spigot-93e20b3-ef2f382" the EntityArmorStand.getHeadRotation()  returns the "yaw" of the armor stand (that's the property yaw of "net.minecraft.server.v1_12_R1.Entity". In the previous versions it just returned 0

The problem however, is that setHeadRotation() sets the property of "aQ", so it appears to me that getHeadRotation should also return the value of "aQ" instead of "yaw", as currently "yaw" could give outdated results. Or maybe "setHeadRotation()" should also update the yaw so "getHeadRotation()" returns the correct value