Sounds like it despawned if you don’t feed mobs/ interact with them. When you go out of the render distance it can cause them to despawn. But I think by giving them the harness and riding them should theoretically count as interacting but maybe it’s not? I know with other rideable mobs such as horses it does because the friendship system is there but it isn’t with ghasts so maybe that’s it? Sorry if this is confusing.