If the cost to make a metal limb is lower than the cost of food then your society has some issues, but putting that aside:
Many insects are willing to and capable of tearing off limbs to escape predators. Spiders, some flies, and even beetles would rather lose a leg and live to fight another day than be caught by a limb and fed to a hungry chick.
So your starving lower castes? It makes perfect sense for them to tear off their own limbs in order to eke out a few more days of life. Not only do they no longer have to supply that limb with nutrients they also get a tasty tasty calorific package, and if the technology exists to replace the limb then why not use it?
Prionic disease is an issue if others are feeding off their limbs (say they sold a leg to a soup kitchen in order to pay the rent that month), but in the cases of autophagy (eating themselves) it's not too much of a problem. This also depends a lot on your creature's reaction to trauma: If losing a limb is likely to make them die of shock or bleed out then this is obviously not a viable survival strategy.
Obviously, if these starving and desperate souls survive the circumstances that drove them to eat themselves, then they might be able to scratch together enough resources to replace their missing limb. Or maybe some kind soul just gave them mechanised limbs and a gun and sent them off to pillage alien colonies.