It could be one of two things:
1.) This could be a very slow-feathering bird (males sometimes do this). Is the bird truely tail-less, no litte nub at all? Or does it just lack feathers on it's tail-bump yet?
2.) The breeder doesn't understand the difference between Araucanas and Ameraucanas, and crossed them. The tail-less gene could very well make it to the resulting mixed offspring.