I noticed this when playing my super sim, who has nearly all the aspiration traits (including Hardly Hungry), in a new save. This only started happening after the last patch. The hunger need depletes...
Ideas
Use the cheat
| traits.remove_trait HardlyHungry |
And then the cheat
| traits.equip_trait HardlyHungry |
And see if it fixes. It sounds like your Sim lost the hidden buff that halts hunger decay. Adding the trait again will add the buff back.
