Isn't it amazing how different the rules for a Oneworld award are throughout Oneworld? Luckily AA has been changing F/Y to J/Y on more routes which helps this situation, all of the longer Caribbean and Latin America routes are now called J instead of F, as well as the routes to Canada and Mexico.

There are a couple of other award types which do this, but I forget which ones precisely. So never assume that U will get you Z. It isn't just a OW thing, both Star and Skyteam also have awards where business downgrades to economy instead of up to first.