Utilitarians don't necessarily think that utilitarianism is a guide to behaviour or assigning moral responsibility. They might even reject the usual sense in which we're concerned about moral responsibility since, if utilitarianism is true, punishment and reward ought to be apportioned based on utility rather than deserts.

At its minimum, utilitarianism is just a theory about what constitutes the good. That it might fail to inform behaviour or judgement certainly looks like a problem at face, but it's not clear that this means we should reject it in this latter sense.

see, e.g., https://plato.stanford.edu/entries/consequentialism/#WhiConActVsExpCon