Tin foil creates hotspots, it doesn't prevent them. If you put the dull side out it helps to diffuse them but doesn't stop them.

I'm going to go with nute burn myself. What kind of soil is that?

I've read that rain water contains Nitrogen in it (I can't confirm if this is true). Which is what your burn looks like (I should know since I seem to be friggin notorious for burning my plants). Also what is the PH of the rain water you're adding to your plant?

If you could also tell us what kind of soil it is and type up the contents of the bag?

I'm no expert but just trying to get more information so the real guru's can help you.