I am a scientist (biology field) and I have been exercising and playing competitive sports for more than 35 years at this point. I used to be a hard-nosed calorie counter and I got fantastic results by counting calories, which means I had visible veins in my belly area. I read Ancel Keys experiments, and I used to say that there were no fat people coming back from war back in the day: calorie reduction works.
I used intermittent fasting, high protein, low-ish carb, daily variation in calories (e.g., 4 days at 1500-1800, 3 days at 2500).

The pandemic starts, I had surgery a month before the working from home mandates, I stopped smoking (5 cigarettes a day, we know they tend to suppress appetite, but it not like meth, there are plenty of fat smokers) and I quickly, say in two months, put on around 10 kg (90 to 100 kg+), and for the first time since I was an early teenager, I had a belly.

I was exercising less, not counting calories at all, I was not spending calories going outside (work, errands, nights out all got cancelled), and I was hungry all the time despite eating more than usual. All factors that can explain my weight gain.

This unfortunate situation goes on for one year more or less, then I started losing some weight by simply eating less and going out more, at least that was my explanation. But I noticed a visible change in body shape and composition after a two-week vacation in which my diet was McDonald's at nights and a huge breakfast in the morning with all calorie-dense food, basically.

Fast forward a few months, some improvement here and there, but I am still not at my pre-pandemic fitness level, still a bit fat, muscles not as visible as I like.
I then go to NYC for a couple of weeks and my diet is McDonald's, ice cream and nothing of the healthy sort. Sure, humidity, heat, walking much more, they can all contribute to non-trivial increase in energy expenditure. But I come back looking like a million bucks, pre-pandemic fitness level if not better.

I keep in shape, then I get covid and I pretty quickly put some weight on (again...), say 5-10 pounds (sure, these swings are also water stored in glycogen, some of the weight quickly gained also goes away pretty quickly). I then go on a long-weekend vacation, walk much more than usual but no exercise (gym, running, combat sports), eat and drink more than usual, and, again, I observe a visible change in my body composition. I am leaner, I lost weight, I see (some) abs.

Someone could say, you are simply exercising more via walking and other non-exercise related energy expenditure, and MCDonald's two times a day can be at < 2k calories, which is plenty deficit for my body to lose weight. But as someone who has been tracking calories for a long time and has exercised for decades, I don't think it explains what I am seeing.

I have problems now accepting that our metabolism and our physiology is not a lot more malleable than I (and science) thought it was. I now start to believe that our "state of mind" (conscious or unconscious) can have a much bigger effect on our physiology, including metabolism, than expected according to "settled science". I am talking about our "mind" perceiving a new state of world (vacation for example) and very quickly adapting to the new perceived "stress" by changing my physiology, it could be different nutrient partitioning (decrease insulin resistance?) or simply higher metabolic rate.
Or it could be the opposite, such as perceived "normal life" as stressful thus increasing, say, insulin resistance, and vacation as "relaxed mode", decreasing cortisol and insulin resistance etc.
We all know we can "think" ourselves sick, but we "think" ourselves leaner?