Unable to complete the Aspirations "Master Mixologist" if player chooses The Master Chef branch

That's just your choice. If you want to complete every single aspiration on one Sim, then you need to plan it, for example you'd need to choose Comedian rather than Entertainer too or Joke Star would be impossible. TS4 is about you making choices, that's not a design flaw. If you wanted to complete the Mixology aspiration, why did you choose Chef? You can complete the Chef aspiration by choosing Mixologist career, so you had no reason to pick it if you cared about the aspirations.