Reported: NPCs disappear at random, sometimes several standing together, and only come back after restarting the game.
Found: The game stops showing anything more than one map block away from you, which can be as close as 32 steps. The server thought NPCs stayed in view out to 50 steps, so when you walked a short way off it did not notice the game had dropped them, and when you came back it never sent them again. Only a fresh login sent them.
Fix: In the next DragonSoul update the server follows the game's own view rule, so NPCs, players, buildings and pets are sent again as soon as you come back near them.