They look ready to harvest. Three possible causes I can think of are — heat stress, fungal infection, or sucking insects. They are closely planted and may not be getting sufficient air circulation. Watch out for slugs.
I would carefully remove all of the wilted and damaged leaves using long, sharp scissors, assess and start harvesting every other head, or — even though these are heading lettuce, you can harvest the outer leaves as they become eating quality. That’s the beauty of home gardening. You can have fresh lettuce harvested right out of the garden minutes before eating.
Have a container of cold water ready and float the lettuce in it as soon as you harvest. My general rule is to rinse three times in sufficient water for them to swim so any grit will float free and sink. Add ice cubes to the final rinse water to crisp them.
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