I have built a few trident killers and this is a recurring problem. You place a trident in a 2 block by 2 block space, enchanted with Impaling 5 using pistons to create contact with mobs. The mobs die after impact with the trident and deliver XP to the player. This all works as it should when I am in the loaded area however when I leave the loaded area and come back the tridents sometimes randomly despawn (approx 1 in 5 times).

An example is in the following video of how I use them https://www.youtube.com/watch?v=Aqf69erGGN8&t=1321s