I had a very similar experience doing this too. Lost about 21kg in 2-3 months and went from 98kg to 77 kg in 3 months which is a lot for a guy who is 5'7. 16/8 IF(Intermittent fasting) , mostly plant based and one day was a cheat day.Didn't exersise at all for the majority of that period and even when I did the intensity wasn't high enough to put a dent in caloric deficit coming for eating less.

I did this 5 years ago and am still doing it and haven't gained my weight back. But I'll say that retaining my weight is harder now.I was 22 when I did that and stabilized at 71/72 kg for couple of years but now at 27 I am stable at 76/77 kg.Although exercising more and maybe having gained some muscle mass might account for that extra weight.My body fat percentage (assessing only visually) seems lowest than it has ever been.