A single-word ḥāl occupies the accusative case, but its visible marker depends on the noun class.
«رَاكِبًا»: singular, fatḥa.
«رَاكِبَيْنِ»: dual, yāʾ.
«مَسْرُورِينَ»: sound masculine plural, yāʾ.
«مَسْرُورَاتٍ»: sound feminine plural, kasra.
Accusative does not always mean a visible fatḥa.