Nute Burn or Potassium Deficiency?

This one has me stumped. I'm leaning towards burn though.

Medium: Coco/Perlite
Nutrients: Terra Aquatica Tipart (recommended feed)
Ph Input: 5.8

The reason I'm leaning towards nutrient burn is because I underestimated the stretch and they had pretty harsh dry backs in the summer heat.

The leaves directly under the light are most affected.

What is the EC of the recommended feed and how often are you feeding? What is the EC of the runoff?

In coco, you should not let the media dryback. The drybacks you had probably caused the EC in the medium to spike. If the top of the coco starts to get light brown, you let it go too long before feeding.

Using an appropriate EC, you should feed at least once a day, and depending on pot size, at least twice a day in flower.