I have a similar problem. Dead Bots get stored in a Storage Zone that has the option to store Hauler and Claning Bots off. When I create a new Zone to haul them to they won't get hauled and I can't prioritize the hauling, because the game says there is no empty spot configured to store them, but there is.

Dead bots are not Hauling Bots, but Hauling Bot (disabled).
Check your settings in Items -> Robots -> Hauling Bot (disabled)