Axolotls dissapear

Hi, I’ve trying to have a little lake with axolotls, frogs and turtles inside in my Minecraft world, but I can’t, because whenever I get too far from that lake, when I come back, the axolotls dissapear, and I don’t really know why, but it seems that only grown up axolotls dissapear, because the baby axolotls are still there when I come back.

Only the grown up axolotls dissapear, the other mobs, like the turtles and frogs that I mentioned at the start, stay there.

So please, if you can, could you fix this bug? Thanks for reading, I hope you fix this quick and that it isn’t too much trouble for you.