Don’t obsess over the scale during your fast, just weigh yourself before and a day after refeeding.
Water, glycogen and fecal matter will go down and come back up. You could still easily lose 5 to 10 lbs of body fat and other tissue during a week long fast. You will lose some fluid and other tissue associated with the body fat too so it isn’t as simple as BMR x days / fat caloric density.
If you’re not already on a low carb diet and depending on your current weight you could easily lose 10+lbs of just water. If I’m not mistaken you should be able to expect around .5lbs of actual fat loss a day. So if you lose 14lbs, then immediately go back to a sad way of eating, you’ll likely regain most of not all of the weight.