The advantages of hydroponics are no pollution, increased indoor humidity and the best ornamental effect. You'd better not eat your own vegetables. If your home has been decorated for more than 3 years, you can eat it.
The situation you described is sunburn. The reason is that lettuce itself is a vegetable without crude fiber. Planting with soil adapts to the earth's environment, with strong sunlight and rich carbohydrates, that is, thick leaves. No matter how the light changes, it has strong resistance.
Your hydroponic lettuce can grow, but the quality is not enough. Its disease resistance. Drought resistance. Strong light resistance, etc. Can't adapt to completion. So with strong light leaves, the end of the film can't get enough water. That is, evaporation is greater than supply. Dead leaves are inevitable. Just like bean sprouts basking in the sun.
The solution is the same as having sunshine without light. You will grow up smoothly.