1.16.100.57 introduces a spawning radius change for simulation range 6 and higher that allows mobs to spawn in a 128 block radius around the player, limited horizontally by the simulation range.
However, chunks will only attempt spawns if the northwest center block of a chunk is within 96 blocks of the player (presumably to prevent the algorithm from being run in far away ticking areas).
This means that despite simulation range 12 leaving enough space horizontally for spawns 128 blocks away, mobs will only spawn in chunks with their center within 96 blocks of the player.

How to reproduce:

1. Set up two repeat command blocks on a superflat world:
   execute @e ~~~ setblock ~ ~-1 ~ stone
   kill @e (delay 1gt)
2. Set the simulation range to 12

Expected results:

A hollow circle forms from mobs placing blocks below them, with a radius of 24-128

Actual results:

Chunks of which the center is past 96 blocks from the player do not spawn any mobs.