In case entity gets spawned by nether portal due to previous entering at another end and if you would cancel CreatureSpawnEvent then entity never spawns at another end, which is correct, but it remains in cache even tho it got marked as dead. After which point removing this entity with usual API will not work and only server restart clears out them. 

This is only issue with 1.16+ servers

Side note, might not even be related, but noticed that entity spawn reason when its spawning from nether portal is set to DEFAULT instead to NETHER_PORTAL. Tho this is the case on older version too which doesn't have main issue, but just interesting note.