An item tag is named "minecraft:fishes", while the plural of fish is "fish"

Where exactly is this item tag?

Pedantic, but fishes is actually a word, in certain cases.

are you sure that this is a bug

Affects 1.13-pre1.

Can confirm in 1.18.1.

Can confirm in 1.19. While both "fishes" and "fish" are acceptable spellings, "fish" is more commonly used and would be expected. You can reproduce this issue by typing the following command and observing the name of the item tag.

/clear @s #minecraft:fishes

It's not a case of "common usage", fish (plural) is used when refering to eg a group of clown fish, fishes is used when referring to multiple fish species at once.

See also pokechu's link.

Yes; after further research, it appears that "fishes" would fit in this context, however, it's Mojang's decision on whether this is a valid problem or not. Can confirm in 1.19.2.