It looks like physical damage to me. Tomato plants at this stage have fairly tender leaves still. If they're bent, bruised or mishandled, the leaf vein could be broken even if the leaf looks intact yet. My guess only as I don't know how the plants are handled.
Don't just water them a little every evening. All that does is encourage the roots to grow near the top of the soil where moisture is. Water them deeply and then don't water them again till the cup feels much lighter in weight. Doing it this way encourages roots to grow deep. As the top layer of soil dries out, the roots must go down to seek more moisture.
Notice the fuzzy hairs on the tomato stem? If you remove lower leaves and transplant your tomato deeper each time, covering the stem with soil, those fuzzy hairs will grow into roots. The more extensive the root system, the more soul nutrients and water they can pull up. The result is stronger plants that bear heavier and are more resistant (though not immune) to watering variation since they can take up more water.
I'd ask first for empty 1 litre (quart) milk cartons from friends and them move them up to 2 litre cartons. I'd transplant my tomato plants a few times before they were planted in the garden. Each time, I removed some bottom leaves and planted deeper. I did the same the last time when they went into the ground except this time, rather than deeper, I planted them sideways. Tomato roots and plants like warmth and if I planted them straight down, it would be too cool for the roots. The leafy part sticking up at an angle straightened up in a few days. When I cut and tore the carton away, roots were visible all the way through, not just at the bottom.
This only works for tomatoes, not peppers since their stems aren't fuzzy.p