I haven't played with either aspiration, but I did some hunting through the tutorials and guides others have written, and it looks like the issue has to do with the two branches of the culinary career.
Master mixologist requirements: Mix 20 Drinks (75 Satisfaction)
Own a Bar and 2 Barstools (50 Satisfaction)
Achieve Level 5 Mixology Skill (300 Satisfaction)
Mix 10 Drinks at a Single Social Event (200 Satisfaction)
Mix 3 Different Excellent Drinks (150 Satisfaction)
Become an Adult (0 Satisfaction)
Join the Mixology Branch of the Culinary Career (500 Satifaction)
Achieve Level 7 in Mixology Skill (500 Satisfaction)
Achieve Level 10 Mixology Skill (1000 Satisfaction)
Mix 10 Excellent Drinks While Inspired (750 Satisfaction)
Create 10 Types of Excellent Drink (1000 Satisfaction)
Master Chef requirements: Cook 5 Excellent Meals (50)
Cook 2 Meals while Inspired (75)
Achieve Level 5 Cooking Skill (300)
Cook a Gourmet Meal (250)
Earn Silver at a Dinner Party Event (250)
Become an Adult (0)
Reach Level 4 in the Culinary Career (500) Create 3 Types of Excellent Food (500)
Achieve Level 5 of Gourmet Cooking Skill (750)
Reach Level 8 of the Culinary Career (1000) Cook 3 Gourmet Dishes at a Single Event (1000)
The Master Mixologist aspiration actually requires the sim to join the mixologist branch of the culinary career. But it looks like the Master Chef aspiration only requires them to get to a certain level (no specification of which branch, so according to this, it shouldn't matter).
Players who started with the Chef aspiration, probably chose the chef branch, and then could not complete the Mixologist aspiration because you can't switch branches. But if you choose the Mixologist branch, it looks like it will still work for the Chef aspiration (at least the way it's worded in the step descriptions).
Here are the sources of information I used, so you can see if you agree with this interpretation:
https://carls-sims-4-guide.com/aspirations/masterchef.phphttps://carls-sims-4-guide.com/aspirations/mastermixologist.phphttps://carls-sims-4-guide.com/careers/culinary/
