Your camera is probably leaving the ABTree's maximum dimensions and then just decides to render nothing as the ABT does not return potentially visible objects. For some reason the ABT is not fully updated automatically in A8 (like when you leave its area with your camera or move certain objects beyond that).
It could be that it's using level_ent for the bbox. .. The beta features page says:
The level size (level_ent.min_x/max_x) was automatically adapted to the maximum coordinate of an object moved or placed in the level. This led to undesired behavior when the level size was set up manually. It is now only adapted when level_ent.max_x/min_x was changed in the script .
Try debugging/ displaying those values. And/ or place a block in various corners of the map, not just in y direction but in z (and x) direction, too.
