No - there's nothing unstable or buggy about HapMax. The utility monster is a large change in the input that causes a large change in the output. Instability is when a small change in the input causes a large change in the output. HapMax is stable by any measure I can think of. You just don't like HapMax because you don't think you implement it.

(If you could really perceive the vast orgasmic pleasure of the monster, rather than just reading a text description of it, you might find that you do implement HapMax.)