When using onjoin: true in spawn.json and loading into a world causes most chunks will not load & entities will be frozen in place.
Then when exiting world it will get stuck on saving world, need to then force kill.

Upon restart MC and load world no chunks will show at all.

Removing the onjoin line then restart MC, load world all fine again.

1.16.4
Forge - 35.1.4
incontrol-1.16-5.1.5

[
  {
    "dimension": "minecraft:overworld",
    "mod": "alexsmobs",
    "onjoin": true,
    "seesky": true,
    "passive": true,
    "result": "default"
  }
]

Adding log but does not seem to show anything
latest.log

This should be already fixed next release (soon)