No sunburn, looks like edema to me. Look underside the leaves and if you notice crystal like white stuff, it is pretty a sure thing that it is edema.
Edema:
This is a condition caused by a chili plant taking on more water through its root system that it can use in growth or transpire through its leaves.
It causes strange-looking crystalline growths on leaves. In itself, edema causes little harm but is symptomatic of other problems.
So in short it is mostly caused by overwatering the chili.