I actually tend to semi-fast on my rest day the day before my long zone 1 workouts, though there are days when I abandon it because I can tell I just need to eat. I can’t imagine there isn’t some fat adaptation going on by just fasting even if it’s not a training day. At the very least I can’t imagine it’s detrimental as long as you make up the calories from the skipped meal at lunch/dinner.
For what it’s worth I can say that in my experience of messing around with fasting while losing weight there was a big psychological benefit to randomly skipping meals. It helped me learn how to tell the difference between wanting to eat because I was actually hungry/low on energy vs wanting to eat just because my stomach felt empty or I was bored. Once I started to get back into backpacking I kept playing with seeing how far I could go on an empty stomach and now following a training plan it’s helped me power through fasted workouts where I can tell I just want to eat to eat not because I’m actually low on energy.