[Neus]

It will only despawn in this way if the boss core has been defeated or if the dungeon has been 'alive' a while. Since this is a necessary function, I don't see many ways to extend it that make sense; increasing the lifespan of the dungeon by 1 in-game hour when it generates a new floor is the only one I can think of at the moment.

It's usually a better idea to carry things like repair powders and such so you don't have to leave the dungeon.