Mobs do not spawn when the world border is very small

I tried a worldborder expanding datapack as a challenge but noticed that mobs didn't spawn, even when there was a mob spawner in the border. I'm not sure of the sizes affected.
Here's a screenshot of me testing this in a completely vanilla world, waiting for more than 5 minutes after killing any nearby entities using a 3 size world border (notice how the Entities counter only says 0/1)