If a character registers in the AOI grid at the same moment an NPC moves between grid cells, that NPC’s spawn message can be missed.
Why Processing an entry, channel change, or teleport happens at the same instant an NPC moves → Effect That NPC is left out of the “newly visible entities” calculation → On screen A few specific NPCs are invisible, or NPCs that already left are still there
Handle AOI updates on one thread in one order, periodically resync the entire “visible list.”
On the graph
Random spikes · Mismatches between the server’s visible list and the client’s entity list
Where to look
Server log of AOI grid registration, entity cell moves, and spawn/despawn message sends with tick numbers, plus periodic comparison of the server’s “visible list” with the list the client holds
Confirmed if
The missing NPC changed cells in the same tick as that character’s entry or teleport, and there’s no record of a spawn message sent for that NPC
Ruled out if
The spawn message was sent but the client didn’t receive it or dropped it: delivery side (spawn data lost in the burst after entering, spawn messages dropped during loading). Always the same NPC missing: phase or display settings difference
Check with
Game server or client logs and metrics
Sources
Replication Graph in Unreal EngineEpic Games MMORPGs and similar games split the world into a grid with an actor list per cell and send based on the cell the client is in
Actor Relevancy in Unreal EngineEpic Games Relevancy is decided per connection, and actors that are no longer relevant are destroyed on the client