By default, Spigot sets the distances from players that monsters and animals become active at to lower values than in Vanilla Minecraft.
This reduces the amount of work that the server has to do but players may notice that animals and monsters are not moving even though they can see them.
If you are running a Spigot server and wish to have it behave more like Vanilla Minecraft then you can edit the
Below are the settings and values you need to change in the file:
entity-activation-range: animals: 64 monsters: 64 misc: 32 entity-tracking-range: players: 144 animals: 48 monsters: 64 misc: 32 other: 64
For information on how to edit the file, check our guide on Editing YAML files.