What is are the umols and PPF rating on your light? I just experienced light burn myself and the leaves near the top look tired and old when it happens and your issue only seems to be near the top. If it's a deficiency possibly zinc and it's simply time for another normal feeding. I do DWC though and am no expert for soil or coco. The other reason I wonder if light burn could be an issue is because when I zoom in I see the edges of your leaves showing mild signs of 'burn' too. If it is light urn it ain't bad and will recover. Let us know if you do nail it down! Good luck!
Also agree on the not stripping of any fan leaves! They act as a buffer in addition to being the main food producer for the plant!
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