Thank you for your question. You don't indicate what type of seedlings you are growing, but it appears you have some which are very healthy. There is a common issue with seedlings called "damping off" which may be the cause of your leaves falling off. Here is a link to a University of MN Extension page which describes this:

https://extension.umn.edu/solve-problem/how-prevent-seedling-damping

Another excellent article addresses potting soil and the need for fertilizing of seedlings which may solve your yellowing problem. I would recommend a light application of fish emulsion fertilizer or others they mention:

https://blog-fruit-vegetable-ipm.extension.umn.edu/2022/04/stunted-seedlings-just-cold-weather-or.html

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