I have the similar problem and it started right after i pointed a fan on those leafs. First I thought they got old and died, they are the lowest leafsets (4th-5th) I'm using bloom nutrients which are poor in phosphorus.

Here are the pictures. They don't get much light but cool air. I lowered the speed of the fan about a day ago. Haven't done anything else yet. Another plant far from the fan having similar old leaves death. This makes me think its not directly connected with fan. I am also suspicious about P deficiency. But still can be connected with air. They got too much air but not enough elements to use, dried out. I am thinking very simple, am I?

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