Strange that first flush is okay, then shit happens - 3 years running.
How certain are you that you don't overwater it?
It is doing great, you reduce the foliage load (= reduce the transpiration rate), keep pouring the water on like you have been .... every year.
Maybe it is hard to believe, but ....
The recurring problem does not seem to be responsive to antifungals = maybe it isn't fungal.
It is the same tap water before as when this occurs as after = no pH, or certainly you should be able to verify/refute with the Water Dept.
etc.
hmmmm