Same thing happened to me on 3 different Foundry maps. Probably wont be fixed until they re-open editing.
I'm not sure if we'll be able to fix on our end, if somehow the npc is set to auto disappear once it's out of range. Every single mission that has a npc that you must return to after a fetch quest is now broken. Every mission with a large site to casually explore... the same.
The only missions not affected are the most linear of all types of Foundry missions. If you have to visit a spot on the map more than once, then it's borked.
I can't imagine a spotlighted mission that isn't plagued by this show-stopper of a bug.
Yeah, I pointed out in one of the other threads here that the enemy mobs in my missions are disappearing when you move too far away from them, just like the friendly NPCs. This is particularly problematic in space battles, when players will frequently "fly away" from the battle in order to recharge their shields, etc. Get a little distance away, and the enemy ships vanish, never to return!