this is a normal behavior on a Servers with the Sponge-API. (1) Among other things, the entity behavior is controlled via the entity-activation-range. (2)
entity-activation-range Default: (animals: 32, monsters: 32, misc: 16) Type: Integer Description: Controls the range in blocks that entities will become "activated" - entities outside of this range will tick at a reduced rate to prevent server lag. When changed, these numbers can adversely affect gameplay, so edit with caution. Lowering these values can give a major boost to performance, but at the cost of affecting gameplay behavior. Lowering these might impact item and monster farms, but should have little impact on normal behavior.