I think the others are right that the feed and other amendments are the problem. I do a lot of indoor growing (micros) but I never use anything more than multipurpose peat compost until the plant needs to go into an 8inch pot (that's a micro's final destination).
But it could also be oedema, because problems can be multifactorial! For your next batch, keep an eye on your lower leaves and if the otherwise healthy plants have corking, find a way to lower the humidity.
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