NPC and Vehicles keep disappearing, and reappearing

[awm]

Just as the title says. One mission I was doing was saving 3 hostages and bring them to a nearby location with a weapon stash. I arrive at the hostage location and I see three of them, a bunch of enemies, and a plane parked by the pier (I think is what you call it). After I rescued one of the hostages, the other two disappeared. I turn around and look for them and the plane is also gone.
This is not the first time it happened. I once saw a parked chopper with two dudes. After deciding to try to steal it, an icon popped up on my screen which is that hostage van thing. I turn around to look at it for a sec, and said "nah f*ck that". As I turn around to continue to steal the chopper, they were all gone, even the chopper.
As for the case for the "reappearing" that I mentioned, an example are those hostage vans and those explosive trucks which spawn at the same road with the same routine everytime. You get out of the area, and then get back, they all respawn and do the same thing.
Same thing with the small skirmishes you do on certain locations, like blockades on bridges and hostage rescues in small villages. One second you obliterate everything, and the second you get out of the area, about 200m away, and go back, they all respawn and do the same thing over again.
I think this is kind of game-breaking.
One more thing, sometimes the game difficulty randomly switches itself to easy. You might have not noticed, but it happened to me everytime I completed missions.