I agree heat/light stress could be the issue, especially since the issue seems to be isolated to the top portions of the plant. However, while adjusting the distance/intensity of the light, be careful that you're not overwatering. It's easy to overwater in soil and even easier in the buckets that you're using. Over- and underwatering often have similar symptoms. Check the weight of the pot you're growing in and see if it feels unusually heavy (or light) and if you're able to, use a moisture meter to see how wet it is at the middle and bottom of the bucket. If you don't have a moisture meter, a little trick is to use a wooden chopstick. Stick it into the soil as far as it will go then pull it out. I've found that soil that is the correct moisture will leave a small amount of soil/moisture on the chopstick but it won't feel wet or muddy, nor on the opposite extreme, it won't stay bone dry.
From the photos, looks like you've caught either potential issue early enough that with patience and gentle corrective action you'll be able to get back on track!