Optical illusion is most likely the reason instead of engine brake if your vehicle slows down significantly in certain areas and doesn’t on other downhills with similar angle.

There is this slope near a waterfall around where I live. If you put your car in neutral gear at the bottom of the slope, it rolls up the hill. We named it magnetic hill back in high school, but I later found out that it was just an optical illusion that happens everywhere around the world. Basically some uphill looks like downhill and vice versa. Google “Gravity Hill”.