When you say very soft water, you mean naturally, not from a water softener, right? Artificially softened water is loaded with sodium, which could do this.
Leaves dying back from the tip usually means insufficient water uptake for the temperature / humidity or too high mineral load in the water. My guesses would be insufficient humidity, excessive heat or not enough water. Did it happen recently, or this summer?
This plant might have different genetics from the others and be more sensitive to issues.
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