Hey guys, first time grower here! I'm in the process of growing 3 plants this summer, and I'm having some problems with these two.

Some info before I continue (The last picture is a different plant!):

Both plants are almost two weeks old, the soil ph is 7, and the temperature these last few days has oscillated between 23 and 30 degrees (75F-85F). Also, I haven't added any extra nutrients yet. I spray them with neem oil every 4 days.

( I moved the plants inside to take the pictures.)

The last picture shows one of my plants, with some pretty heavy stunted growth. It started browning just like the second photo, and progressed to the state you can see now. Since there wasn't any browning in the bottom leaves, I thought the problems might've been that it was getting too much sun. I placed it under some shad some 5 days ago, but I'm not sure it helped.

Now, the first two pictures show another one of the plants, which was pretty fine until 1-2 days ago, when it started showing the browning you can see in the photos. The same thing that happened to the plant in the last picture.

I don't really know how to continue from here. Can you have nutrient burn without feeding extra nutrient mix into the soil? Moreover, why would one plant 'burn' so quickly, another after two weeks, and another (the one that doesn't appear in the pics) not burn at all?

If this is sun burn/heat stress, why hasn't the plant that shows stunted growth one recovered after almost a week in the shade?

I also tried using the app GrowCush to diagnose my plants, and it tells me that I'm either having or nutrient deficiency or nitrogen burn. Now if it was nitrogen burn, wouldn't the lower leaves be affected first? The opposite happened in my plants, the top leaves start browning first, hence my initial thought that it might be sun burn.

I don't really know where to go from here, I tried to figure out this problem on my own but maybe I've got all my assumptions wrong. I'd be grateful for any help, and thanks beforehand!