I’ve been trying to beat Minecraft in an Ocean Buffet. This teaches you a few interesting things. Most recently, because of lack of cattle in the Minecraft Ocean Biome, I’ve taken to fishing to find leather to make books to make an enchanting table and the requisite library.
This brings us to leveling up Fishing poles. It appears (after many hours of fishing in Minecraft) that Luck of the Sea actually reduces your chance of getting leather, while increasing your chance of getting enchanted books.
As close as I can tell, when fishing, you have categories of items that you might get. Fish is the primary item, but their are other items like boots, string, and leather. When you have Luck of the Sea, it appears that catching an item in those categories results in a chance of an upgrade to a better item in that category. For string it might be more string, for boots it might be enchanted boots. And for leather, it is likely an enchanted book.
So the moral of the story is, if you need to fish for leather, don’t put Luck of the Sea on your fishing pole in Java Minecraft 1.13.2.