I tested it as well, and as you can see in the
first replay by Chansey being OHKO'd by Stomping Tantrum after failing Fissure, it does work as intented, meaning you fail Fissure and the Stomping Tantrum's power doubles.
However you mentioned that it "should" say something else than IMMUNE, for whatever reason. So i tested whether or not it would work after failing a move due to the opponents being immune to said move. By looking at that
second replay, you can see it works the same. So i really don't see why saying IMMUNE instead of FAILED or something else is a bug.