Once had a similar problem where everything would vanish if I fell too far in my platformer.
It happens when the camera leaves the level(_ent) bounding box as Reconnoiter has already suggested with his fix. The ABTree is spanned and calculated across that box, so when the camera leaves it any query for possibly visible objects gets terminated quickly.
There has been a change at some point to automatic updates of level_ent in relation to (dynamic) entity positions, see "bugs" or "beta features".