I just can't imagine this has anything to do water. I use city water, not only on plumeria but hundreds of other tropical plants, and have for decades. I see nothing like what your pictures show. It also does not appears to be fertilizer related either. My tropical plants are fertilized far more than what you have done. I do use an magnesium solution on my plants pretty regularly.
So, that being said, I'm not much help to you. I don't know what the problem is.
