That's interesting, I think I've had a plant do the same thing before. Don't quote me but I think it's from regular water. This is a calcium based line, not nitrogen, and when you take that cal away and the plant makes cells with out the constant cal uptake the cells are week and deficient. Its not burn, or deficiency, but a rupture of a week cell wall, n they dry n rot fast. I never use plain water n never saw the problem come back.
But that's just my take from my research... Hope u get something out of it......
Did you try to flush with just water before the problem started? Did it get super bad like 2 3 days after giving just water?