I grew tomato seedlings of over a dozen varieties under LED’s running 24/7 for two years in a row. No issues with any particular strain, though there are big differences. I wish everything grew like sungold and yellow pear. This is year three, and I have a whole bunch doing that right now in my greenhouse- natural light all day then I run the LED’s all night.
The last two years I’ve done this with two different rootstocks and a dozen different scions with no problems. As a biologist who studies plants for a living I can promise you that plants don’t need darkness when they are young. Something is going on with your seedlings but it’s not the hours of darkeness.
These are my plants about to be grafted. ‘DRO’ rootstock on the left, Brandywind (Suddeth) on the right. Started all the seeds at the same time. I reduced the vigor of the more precocious DRO by clipping off the cotyledons once the true leaves showed, then clipping a few leaves here and there when they first came up. This allowed the Brandywine to cach up. They were pretty much a perfect match for grafting. Growing them paired up like I did this season made my life very easy.
The burned leaves you see is because I blew a fuse and my greenhouse got up to 115 degrees while I was at work due to the roof vent not opening.