I have noticed set points too. Once I break through, then I find a new set point.
I used to be 4-8% BF without trying or dieting (ages 0-30+). Once I tried bulking for real, I took my BF to 15-16%. When I tried to lower my BF, I got stuck at 12%. Took a huge effort to get back to 8%. Ever since then, I easily shoot up to 10-11%.
Weight loss/weight gain is all about hormonal triggers. Protein, fat, carbs are all meaningless except in how they trigger a hormone response. Consider...
Carbs = insulin spike (storage mode)
Fats = precursor to and production of testosterone
In addition to training and diet impact on hormones, our hormone profiles are ever-changing as we age...