I have been home roasting for years using a stovetop popcorn kettle. I had success for so long, and I roast between 200 to 300 grams of coffee
I noticed that the setting for the grinder doesn't have to be tight just after ground the coffee and resting it.
The days passed, and I have to set the grinder a little tighter almost every day.
I bought an HG One used one, and I can feel the coffee changing every day. If the grinder is hard to use then the setting is too open (coarse), If the grinder is smooth then I know I have the right setting. If it is too coarse, then big chunks go between the burrs, and they are more difficult to work. The finer setting, small pieces of coffee are between the burrs, and they are easy to grind.
I have a new Bezzera Strega that it likes fine ground coffee, I can see the shot going faster if I didn't change the setting.
The HG One doesn't slip; I have marks and the marks still in the same position.
I am using 18 grams every time per shot.

My guess is the coffee releases some gas for days and the density changes.

Is my assumption correct?

Yes, one has to gradually change the grind as the beans age. Are you waiting long enough after a fresh roast? The effect is most pronounced with freshly roasted beans. I usually wait a minimum of two days, but notice most beans improve to my taste gradually over about a week. Some monsooned beans are nearly undrinkable for at least a week (funky taste and smell), then they're great for a week or so. The inevitable down turn to my taste buds usually comes around the third week, YRMV. Generally, I try to roast a week's worth each week and stay a week ahead.

The quantity of gas released after roasting has a neglibible mass. The changes in coffee after roasting are due to some compounds reacting with air and water and to some evaporating. In my experience light to medium roast has less nasty tasting things in it immediately after roasting than beans that have been roasted to near cremation.