The biggest problem with CICO is that it solely focuses on calories. Your body needs 2 things to sustain itself: calories for energy and nutrients for your body to actually perform its intended functions. If you’re eating at a deficit, and not making sure you get the nutrients you need with the calories you do eat, then your malnourished body is going to respond in really unexpected ways. Make sure your getting proper nutrition.
Starvation mode is not a thing. It’s a myth created by a society that wants to enable people to stay obese and sell programs, medication and diet plans. If starvation mode was a thing then people with eating disorders would not be skeletal. Our bodies would adapt to survive on for 500 cal a day and people would literally not be starving to death in certain countries
Intermittent fasting puts some stress on the body already, so you may be over stressing it. What happens when the body can’t sustain the energy it’s using is the metabolism will slow down and your BMR will lower. I’d either reduce your exercise or increase your caloric intake. Strength training helps increase BMR as well