Upon loading of world (Minecraft program started just seconds before), player spawns roughly 100 blocks above ground (2-3 seconds of falling), hits ground, and dies. Location of last save is nowhere near where player spawned in air (see attached image for visual explanation).

After respawning, and trying to find location of death (to collect items), player hits an invisible and untouchable wall in the middle of terrain, within ~30 blocks of death. Invisible and untouchable wall behaves exactly like "edge of world" on limited (non-infinite) worlds.

Fresh restart of program fixed all mentioned issues.