Gh is is fine as long as it's above 1-2 of so, it can be 10 or so without issues with so called soft water plants.

KH is the real issue.

YME,

either there is something in you tap water that is not being tested, such as copper etc, or it's just maintenance and consistency.

Sometimes folks have to work and bit harder to get things going, then they can relax a bit after getting over a hump. It could be something like poor/reduced flow from a filter, cleaning the CO2 diffuser, a good trim, not doing a water change when you should , not dosing consistently etc.

I use to think all sorts of things in the past, but I know it's just me not doing or looking at something that might be the reason.

Everytime.

While copper or other issues(high Na+) do occur, they have only been found definitely in 2 or 3 cases out thousands that I've helped.
So they are very rare.

SAE's will nibble R wallichii also.

regards,
Tom Barr