The 1.16.0.61 beta introduces scaled instant despawn radius based on simulation distance. The radii for simulation distances 6 and 8 appear to be consistent with the forumula

(simulation_distance * 16) - 10

so that the instant despawn radius for sim 8 is 118, and the instant despawn radius for sim 6 is 86. This is consistent with the previous instant despawn radius since 1.16.0.51 beta being 54 based on the minimum simulation distance of 4. However, in 1.16.0.61, the instant despawn radius for sim 4 is 44 instead of 54. This appears to be a typo because it is inconsistent with the others, and because it causes mobs to be instantly despawned after they are spawned between r44 and r54.

Steps to reproduce
1. Set world simulation distance to 4
2. Allow a mob to spawn naturally and do not interact with it.
3. Move 44 blocks away from the mob.

Expected result
The mob stays in existence.

Actual result
The mob instantly despawns.

The attached test world contains a non-persistent creeper trapped at 0, 110, 0, and colored glass at Z = 44, 54, 86, and 118 to facilitate testing different instant despawn radii. Change the simulation distance of the world before opening it to test a particular distance.