Mobs do not spawn when world border is small

i loaded a world and set the worldboarder to 16 blocks and no mobs were able to spawn

What I expected to happen was...:
i expected the caves to be filled with mobs which was not the case

What actually happened was...:
little to no mobs did spawn

Steps to Reproduce:
1. set worldboarder to a small amount

2. kill all mobs to make sure mobs actually did not spawn

3. finally check using spectator mod 

 

 

i uploaded 2 videos 1 in 1.21.1 the latest version of minecraft and 1 in 1.16.1 

i showed you how it works now and how it should work