That's because they are UNITCOMBAT_ARCHER not UNITCOMBAT_MOUNTED as they used to be.
Civ5Buildings.xml confirms.
Affected units are Chariot Archer, Indian warelephant, Egyptian war chariot, Arabian camel archer, Keshik.
They were probably changed to unitcombat_archer to get rid of spearmen getting a bonus vs their ranged attack. However, as a result this broke them for the stable.
Civ5Buildings.xml confirms.
Code:
<Building_UnitCombatProductionModifiers>
<Row>
<BuildingType>BUILDING_STABLE</BuildingType>
<UnitCombatType>UNITCOMBAT_MOUNTED</UnitCombatType>
<Modifier>25</Modifier>
</Row>
</Building_UnitCombatProductionModifiers>
Affected units are Chariot Archer, Indian warelephant, Egyptian war chariot, Arabian camel archer, Keshik.
They were probably changed to unitcombat_archer to get rid of spearmen getting a bonus vs their ranged attack. However, as a result this broke them for the stable.