If you run too far or skip areas in Nightfalls you can make champions despawn. If you make champions despawn it counts as not killing them and therefore you cannot get platinum. If you dont get platinum you do not get ANY nightfall rewards. No weapon, no cores, no prisms, no shards, NOTHING.
PLEASE STOP!!!!!
We are not playing these Nightfalls for the end of strike chest for 3 legendary shards. We are doing them for the WEAPONS.
They don't even have to despawn for them not to count towards platinum.
For instance if 2 teammates stay in the 1 room and kill champions, but the 3rd person runs into the hall to the next area. The champions won't despawn, however the game won't count the kills.
I've experimented and it seems, that if you go around 60 meters past a champions spawn point, they won't despawn however they also won't count towards platinum.
Devils lair is another good example, If one teammate runs to the hall before killing the 1st overload champion the game won't count it but the champion won't despawn.