Very interesting. I have not seen anything like this before it looks like a arrow shaped discoloration on the fingers of the fan leaves but only till about half way up the finger before it comes to a point.
I would try and correct your PH first and see if this helps it improve at all. 6.0 is kinda low for a soil grow. I would aim to keep that between 6.4-6.8. a bit higher is ok as well up to 7.0.
Other photos of the plants make it look they are being overwatered, the leaves a curling down and looking droopy, bit different from nitrogen tox claw or dehydration droop.
Best of Luck!
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