I couldn't find any mention of this anywhere so I am assuming this is a bug.

In 1.13, any time a Trident enchanted with Loyalty(any level) was returning to it's owner after being thrown, it would have a visual effect of a blue lasso/ribbon bulling the trident back towards the player.

The last time this visual effect was seen was 1.13, it has been missing since 18w30a. I looked everywhere to see if this was a documented change but found nothing.
This is what the visual effect looks like: