I noticed this on some on my Carolina Reaper plants. A lot of the leaves have this kind of spots surrounding the midrib and the veins.
The plants grow indoors in 5l pots but I kept them in the garden for a period of about half a year when the temperatures were higher.
Some of the spots can be seen on the underside of the leaf as well.
Here's a picture showing the distribution of the spots on a leaf's upper surface.
What am I dealing with here? Some kind of pest? A disease? Or maybe the plants are lacking something?
Bacterial leaf spot comes to mind but I;ve never actually seen it and the pictures I can find online look somewhat different.