If your characters are swaping from Meele to Ranged weapons, i would recomend check hotkeys (i believe it is "Toggle weapon set"), since you are probably pressing something.
(Or you just find some new and by the sounds of it pretty anoying bug)
There is no way to "prevent weapons from being sheated" ... but you can "Toggle Combat Mode" that unsheats them ...
They will still sheat weapons for climbing ladders and stuff, but most of time they keep them out ... it looks especialy good with staves, spears and halberds. :3
