To add in, current ranges I tested are:
4 - 44
6 - 81
8 - 113
10 & 12 - 128
Except the 128 the other numbers seem very random.
Can confirm, I made a video testing all of the despawning ranges: https://youtu.be/vZCa7KcjT6I
Confirmed in the latest beta 1.16.59
Still present in newest beta of April 15th, 2020 1.16.0.57
@Gruva That's not what happens on Java, just like was added to Bedrock in Java mobs have a 1/800 chance to despawn each tick, mobs in Java farther than 44 blocks do not spawn to immediately despawn each and every time.