Can be observed if you set world border to ~300-3000 blocks, fill the void in the nether and place a platform 16x16 at 0 70 xz. Place /worldborder center to 0 500 and little to no mobs spawn. Then do /worldborder center 0 0 and observe how much more mobs are spawning. Additional test showed that the issue appears reliably with /worldborder set 300 and smaller