Certain chunks are outright incapable of spawning mobs. These chunks appear in groups, rather than being isolated. Attached is a screenshot displaying several fenced-in chunks, the ones marked with Netherrack (red rock) are capable of spawning mobs while the chunks marked with Glowstone cannot spawn any mobs at all. Height in the Y axis has no effect. This affects older worlds as well as new; it is likely responsible for numerous mob farms breaking. Passive, hostile, and environmental mobs are all affected.

Pack-spawning may also have an increased effect; leaving only a small area for spawning reduces spawn rate significantly more than expected. Even the fence I put in drastically reduced the amount of mobs that would spawn.