In my experience, its just the stem struggling under the weight of the new forming leaves
I think if you leave your plant to struggle through this process, it will help your plant be stronger later in life
if you see it falling over completely, get a strong but small stick and tie your seedling to the stick for support
Its streched?.. buring the stem down deeper,..and i always get strange wierd mutated leaves on my seedlings, they always grow out of it..
i wouldnt worry, but if theur streched, buring them down and get the light closer