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That is not water burn, rainwater on plants does not burn leaves, chemicals you put in the water is what burns leaves in the sun or under lights, what you have there is an iron deficiency

Anytime it rains, place em in a spot where they get more shade and a little sun. What happened is they got scorched. During the heat of the day when the direct sunlight hits em while they're wet, the evaporation process will scorch em because it dries them out so fast. All you have to do is after it rains, move em out of the direct sunlight to an area where they have mostly shade and little sun. That'll allow em to dry off without scorching em.

Check your pH, the rain could of put your pH out of balance, it could of washed all the nutrients out of the soil so check the ppm if you can, or it could just be from overwatering. A lot of rain can do strange things to plants. Overwatered plants can show false symptoms sometimes.