Zombie nautiluses not spawning

Zombie nautiluses spawn by this rule – drowneds, with tridents in their main hand, have a 50% chance of spawning with a zombie nautilus. So basically, about half of drowneds with a trident spawn riding a zombie nautilus. I have encountered well over a dozen of drowneds with a trident in their main hand, but I am yet to find a zombie nautilus. Apparently, many players have encountered this problem, so this is clearly a bug.