About a week ago we went from our typical spring, wet, cloudy and cool weather to two days of really hot blazing sun in the upper eighties/ low nineties. Gah....felt hotter that it really was, I didn't adapt well either. LOL

Your second photo looks like that might be what happened. Sun burned because the change was so sudden and drastic. Same as if one brought new seedlings out of a nice humid, coolish green house, plopped the babies in hot sun and just left them there. This is the part where reading about harding plants off is the correct chapter.

Obviously with plants growing outdoors already, the solution there is noticing ( being there in the middle of the day) and putting up a shade cover temporally, since once the plants have a chance to adapt slowly, they would be fine with most weather changes.

The 'hot flash' that happened in my back garden dropped several established perennials and my hydrangea on those days . They were still to new after the winter dormancy and freshly emerged so couldn't keep water pumped up in their leaves. This happens in this particular spot each year we get an unusual heat spell too early. So I did remember to grab all the babies in pots ( lucky I happened to have that day off ) and move them to the shade, and they came through just fine. The plants that did get the worst of it are fine now, a little damage on the edge of a few leaves, but we have a whole season for new growth and now everything will have more time to adjust to sun increases gradually. Our clouds are back and the auto-watering system from the sky is working just fine. ;-)

If that's what happened to your cute little lettuce there, I would expect the new growth to be fine and no long term concerns on this front..

The first photo looks like something is munching on the leaves ( more that the more minor sun damage). Baby slugs ?
Or what are your more common garden munchers over on your side of the state ?