I have several varieties of pepper plants and about three fourths of the plants have curling leaves.
What Causes Leaf Curl on Pepper Plants?
Certified GKH Gardening Expert
As with tomato plants, peppers can also suffer from a virus known as leaf curl. This may be what your pepper plants are experiencing, though it could also be attributed to a lack of water or restricted root growth. The following article should help: https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-leaves-curling.htm