Yeah, I think the ability to add legendary fish to ponds was only added around the last update.

Honestly I haven’t messed with ponds too much, they seem like a time and resource sink for barely anything in return, but I don’t think that the fish could “look happy” with only one in the pond before this. But there is only one of each legendary.

So the dev (or devs, I hear he has a small team now) obviously just added legendaries to the list, without realising they now need a new language string to go along with only one fish looking happy as well.

Understandable bug and I imagine it’ll be fixed within the next few updates.