The problem partly is in utility functions being used as both: a) as a metaphor and b) as an exact mathematical tool with exact properties.

a) can be used to elucidate terminal values in a discussion or to structure and focus a discussion away from vague concepts in ethics. But as a metaphor it cannot be used to derive anything with strength. b) on the other hand can strictly only be used where the preconditions are satisfied. Mixing a) and b) means committing the mathematical fallacy: Believing that to have formulated something in an exact way solves the issue in practice.