What BMI was designed for
Body Mass Index was developed as a population-level screening tool to flag broad categories of health risk across large groups -- not as a precise instrument for tracking one individual's body composition changes over time.
What it doesn't measure
BMI cannot distinguish muscle from fat, doesn't account for frame size or muscle-heavy body types, and treats a 220 lb bodybuilder and a 220 lb sedentary individual of the same height identically.
Why this matters during a weight-loss or recomposition phase
Someone gaining muscle while losing fat -- body recomposition -- may see BMI barely move, or even tick upward briefly, despite genuinely favorable changes in body composition.
Better complements to BMI
Waist circumference, progress photos, how clothes fit, and (where available) body-composition measurements give a fuller picture than BMI alone, particularly for anyone doing resistance training alongside a calorie deficit.