1. You don't necessarily need a mod to recover the parts, if you time launch and orbital insertion correctly. But you need a probe core in order to land booster stages (of course, unless you use parachutes) and the game despawns any craft with a probe outside 30km (I think). I'd recommend Flight Manager for Reusable Stages to land stages.
2. No, it's not a new thing; it's been in the game for ages. The game will remove any control-able ships within 30 km of your craft when you're in an atmosphere. This won't happen in space though. Could be that those parts didn't have a probe core and just used parachutes to land. I'm not really sure.