One thing that happens to many people during the Water Fast is that they improve health of their liver and health of their intestines. Digestion becomes more efficient.
Also, their body is more likely to go into winter mode, into storing fat for later, in case there is no food later.
Consequence of all this is faster weight gain on the same diet, after a water fast.
And consequence of this is weight gain, even more than before the water fast.
That is the reason why Water Fasting is not recommended as a sustainable way to lose weight, as it often leads to exactly opposite, weight gain due to more efficient digestion.
To lose weight after a water fast, or to keep it on the same lower level, you must be prepared for permanent change in diet. Like less or no grains, less or no milk, no sugar, low carb etc.
If your goal is just to lose weight quickly by water fasting, and then go back to the same diet, on the long run you risk ending with total weight gain.