Brown marks on leaves and drooping, sad plant

A water drop burnt would have been different , and the burnt is clearly visible when it was caused by a water drop. 1st thing when you can see that kind of spots is to check under the leaves because it could be a bug hidding under ( in your case seems not to be but it's a free tips) In your case and with young plants like this could be anything , but i can see the edges of the leaves curling a bit up due to your extremly low humidity range as 30% is good for dryng maybe but not for growing a young seedling , at that stage you needs 70% at least so 40% more than your actual value which makes me say that is a combo between the low RH and the medium beeing pre enriched ( pre enriched medium can be hot for the young plants and create some excess before thet can recover , the distorded leaves is the 1st symptom )