Mob Entities appear to Spawn then Despawn rapidly in Spawn Area

I just updated my server from 1.15.2 to 20w06a an i noticed while some mobs spawn an stay spawned in, quite a lot of them appear to spawn an despawn instantly as shown in the brief video link. https://puu.sh/F84pl/b275218ae6.mp4

When visiting diff biomes and areas outside of the spawn radius, mob spawnage seems to proceed as normally intended