normal growing variety or stress?

Can't think what else what cause clawing like that unless disease. Best of luck.

Nitrogen content of ammonium too high. temps too low to do the conversion from ammonium to nitrates.

Lots of things. Little things. You have not fertilized yet, Are growing with organic amendments within the soil or do you plan to add concentrates? If there no nutriet in the medium the plant can't grow. Tall fabric pots with lots of perlite. Gravity pulls water to bottom of pot rapidly. Lights at 20cm evaporate top 2-3 inches of medium. Tiny seedling have tiny roots. Nutrient only uptake where water is found. Water is either evaporated or uptake by roots. Fabric pots are not as airy as they are advertised. Constant watering in 65% rh conditions will leave moisture sitting within pot out of reach of roots. The lip of the grow bag is 3-4 inches higher than the medium. Air blowing vertical to it will not make it or very little co2 will even make it to where it needs to be to grow. Night temps you say are 68? Day temps in the high 80's ? Over a 20 degree difference day/night can be Factors Affecting Ammonium (NH4) Levels Certain conditions like low temperatures, waterlogged media, and low pH can prevent bacteria from converting ammonium. This can lead to toxic levels of ammonium in the media, causing damage to plant cells. Symptoms Upward or downward curling of lower leaves depending on plant species; and yellowing between the veins of older leaves which can progress to cell death. When it comes to nitrogen breakdown of a nutrient solution, it’s crucial not to exceed 30% of the total nitrogen as ammoniacal nitrogen. Higher levels can lead to toxicity, severe damage, and even plant death.