Im having the exact same problem but with bell and pimento pepper. But I noticed that the ones that had that leaf curling were only the ones I transplanted. I grew them from seeds and was transplanting them at different stages to see what would happen. (Its my first time growing peppers) and I noticed that the ones that remained in the nursery did not curl but over half of the transplanted ones curled up at some point. A few died but most survived and regained their normal leaves. But I did notice that they seemed to have taken a pause in growing tall and they are just increasing the number of leaves.

Did you find our what exactly is the cause of the leaf rolling up?

What G M described sounds like transplant shock.

E P, was your weather hotter than usual and/or did you change your watering of the peppers? Stress from too little or too much water could cause this. Generally it resolves when the water issue does.

This issue started with some of the plants before they were transplanted outside, while they were still in the grow room. My water regiment has been consistent, although the weather where I live hasn’t been; there’s been some pretty wild temperature swings. None of the plants have died, about 90% of them are producing peppers, they’ve been in the ground for about 9 or 10 weeks, and the leaves still look like this.

After doing a bit of research it certainly looks like water stress. Or poor soil quality (too much clay). Also immobile nutrient deficiencies like Ca, Cu, or Zn could Contribute, and also explain why the older leaves never recovered.

I'm not sure what you mean by "classic" leaf curl? What's classic?

Leaf curl can be caused by a bunch of different things. If newly transplanted, it might be transplant shock. If they are established plants it could be a few things. How long have your plants been in the ground?

The leaves don't look like bug attacks of various sorts. It could be over/under/uneven watering. Or drainage issues. Or fert issues or light issues. Could be heat issues- heat waves can cause it.

It can kind of depend a bit on the peppers. All of them seem to like pretty much the same conditions, but some of them prefer a bit other. Black Hungarians prefer a bit of afternoon shade. Jalapeño types tend to dislike wet feet more and thrive a bit more dry, while Aji types often seem to tolerate wet feet more and don't like it dry at all.

Sometimes as the season goes on some of the most mature leaves seem to tend to just drop a bit too.

Where in the world are these peppers? (General area/big city/part of a state) Zone, climate, and growing season might be relevant to this. 9 or 10 weeks, so they were planted at the end of June or beginning of July? When were the seeds started? What were the plants in before transplanting into ground? (Type of container? Potting mix?) What was their climate in the "grow room"? Is this in an air conditioned house? A room that floats with outside temperature? Basement that's pretty consistantes cool?