I will bet that the nut you refer to is made of plastic, as is the male threaded float valve assembly. In the old days these were made of brass. There is nothing wrong with the newer plastic designs, except that they are very sensitive to over-tightening during installation. I’m sure that American Standard’s installation instructions caution against over-tightening, and probably most of the leaks you refer to are due to faulty installation. If AS didn’t provide such a caution in their instructions, you may have a case.