The Doylist explanation for Zone of Truth banishing the Hunger once Merle and Pan reconnect is simply that Zone of Truth has become Merle’s signature spell, and Griffin couldn’t NOT let it work.
But there’s a Watsonian explanation, too.
The Hunger is an entire plane of existence which has completely given over to despair and the belief that nothing matters. When Zone of Truth is active, no one can tell a lie within its area of effect.
Basically? The Hunger, the physical embodiment of despair, despite being real, is a lie.
The idea that love and heartache and struggle and victory and peace and work and laughter don’t matter is a lie.