It's true that substances in the water can be a problem. That's especially true for oils that change how water droplets adhere to the leaf surface. Substances that dissolve and are absorbed by the leaf, however, typically don't have that effect. Burn happens when the water spreads at the point of contact with the leaf, forming a hemispherical lens that focuses light at a single spot on the leaf. Cannabis leaves have tiny hair-like structures on the leaf that prevent the water from spreading.
Not in my case. I've researched this subject. I've tested it on my own plants, too. So far, I've never been able to produce a burn from spraying my cannabis plants.
It's never rained in any of my tents, day or night. Humidity increases when the lights are off and the tent cools, though.