Not sure if it's a bug or not but it does give us even more of a reason to revisit places after so many days in game passed. I liked how in Skyrim enemies would respawn, but unless you had a quest that brought you back there wasn't much a reason to actually revisit it. Maybe they wanted to give us more incentive to revisit stuff? I've only come across respawning ammo, misc, and aid. I haven't seen any notable high value weapons respawn. I guess that would be a good way to tell if it's a bug or not. If the same fatboy or unique weapons are there again then that just doesn't seem right since it would be easily exploitable.