Not something I've seen before sorry, you might have to go with the less is more theory for now. Maybe look a nutrients or pH or something. If it's only on the bottom leaves it might still grow out of it. Sometimes up-potting helps, it looks to be out of my knowledge range...
That leaf has a...
Are you keeping the moisture level consistent by checking the pot weight? That could be a sign of overly wet roots
Do you have a pic? I had some trouble with watering and changed to hempy buckets with coco to simplify matters, never looked back :)
This is exactly what I do :)
I read you can go up to 1 tablespoon/gallon for initial spray but not too much on chunky buds as water sprays can cause rot
Epsom salt, also known as magnesium sulfate can be purchased at your local supermarket, my wife sometimes uses a whole pack in the bath as bath...
They look pretty healthy apart from the leaf coloring, a foliar spray with Epsom Salt won't hurt and may fix your issue :)
There are a number of possible contributing factors like air flow, soil/medium, pH, lights, temp, humidity etc
If it's just the bottom leaves I wouldn't worry too much assuming your pH is within limits
You can check the soil dampness by lifting the pot to check if it's caused by over watering
You can also foliar spray with Epsom salt @1 heaped teaspoon/gallon which is pretty safe to help boost magnesium...
How is the weight of the pots? not waterlogged?
You can flush easy in coco if needed and just use the basic canna coco A and B, maybe look at foliar spraying with Epsom Salts later...
Are humidity and temps all good?
Lights not too bright or close?