A pic of the full plant sure will help.

But its a very strange symthom. Those spots are so white...

for sure its not a mayor element lacking. And its rare a micro deff shows first on the bottom. And those that do, dont match with those white spots.

I believe its probably related to have shocked your plants by watering with cold water. But ive never seen that symthom before. So if still continues advancing after using ambient temp water, i think we may think on a rare toxicity.