If someone gets different results, that can only mean they changed their answers. You can’t blame a test which asks about preferences for fickleness; you can only note that such fickleness decreases the accuracy (which is an entirely different measure than validity).
They do change their answers, that’s part of the problem. The questions are vague enough and subject enough to the whims of your mood that your answers won’t stay the same from day to day. And that’s most definitely a problem with the test. A good psych exam a person will answer the questions the same way over and over and over (barring long term changes in personality of course) because the questions are focused on core personality traits that don’t change. Simply how much sleep you have will change your MB results, go take the test now and retake it right before bed time, you will probably have different results. Which makes it very much NOT a personality test.