im assuming your soldiers are the adventurers, it wouldn't hurt making them all vampires to fix this via adventure mode trading mechanic w/ goblet full of vampire blood would it?
also, there is no guide for this yet but are you making sure they become "militia-dwarf" by talking to an active duty captain? you do this before going to fortress mode. its the same as becoming a lieutenant or a hearthperson, that would probably help prevent them from being deterred to eat in the fort. your obviously undergoing some sort of bug
since you know, singular adventurers after all, in the world are recognized as "forces from an unknown civilization" if they are outsiders originally. they will also act as an army, if they are ever asked about their profession they will straight up say they are a soldier. but that's not supposed to put them in the place of having effects of hunger as if they are still under player control. they should integrate easily if they don't belong to any civilization, you can achieve not joining any civilization by retiring at some vault or necromancer's tower. you wont have to worry about this if they are originating from your fortress.
but lets say by outside you actually mean like other civilizations, so if your adventurer is from another place other than being an outsider I guess with the new mechanics in place they really don't integrate in and die for some reason instead of becoming "hostile" or an outcast, the dual membership of civs is bugged anyways and they will run off to the outside world because of some position they inherit.
this was probably alot of unneeded info but its what i made sense of how this works during my gameplay and will always affect you no matter what during late game, your going to have to go with my first option unfortunately and hope that they dont become "lord/lady" of some place nobody cares about. i'll edit this if its not right.