I searched for J class using JAL and the results include all other classes as well.
The class selection while searching is for if the carrier’s site requires it and JL does require it. The app just returns whatever is shown on the results page which sometimes includes more than just the class you search for which JL does. You can use the filters to filter out the unwanted cabins.
AC on the other hand doesn’t require a cabin selection but returns results for all cabins.