In my experience in most cases when Acropora react this way I would suspect high alkalinity or low phosphate concentration. Maybe meanwhile I would also add low iodine concentration.
Since reef aquaria are living and dynamic systems this may occur at every age of the corals. Maybe a salt mix has contributed to this, maybe not. From this single observation, even if the problems are caused by the salt mix, the salt mix could be better or worse in its parameters.
Let's say phosphate was at its lower limit and other elements like for example iodine or nickel where limiting also, "improving" the conditions by adding iodine and nickel and releasing the corals from this limitation may finally depress phosphate concentrations further, causing damage to the corals. A lower phosphate concentration in the salt mix may even contribute to the problem although phosphate concentration should be as low as possible, of course.
In reef aquaria, things are only very rarely simple, usually they are complex. This makes the search for causes difficult.
In a reef tank well stocked with SPS, every day of dosing Ca, KH and trace elements may do more to stabilize or destabilize the water parameters than a 10 % water change.
Of course the saltwater is the basis for reefing, nevertheless the impact of a single or a few water changes of 10 % each should not be overestimated. It is more the stabilizing long-term effect that counts.
I agree, and corals will do well even with 0.5 ppm.
I think I have seen much more corals dying from low phosphate concentrations and know more tanks that have been crashed by measures against high phosphate concentrations than I have seen problems with high phosphate concentrations.
I think BRS is a very good and very professional shop and I don't think that. I am sure they are not causing the problem, if there is one with the salt.