I've noticed every now and then that the NPCs in levels were unloading. I researched it a bit and noticed that when standing idle for a specific period of time, the NPCs in a level unload until you move. Most of the times that this happened was when I had another window opened with graal in the background.
The picture below shows what happened after coming back from the trampoline at my neighbors. I had RC as my current window while I was gone.