Another possibility is outside chemicals. Did you have your carpets cleaned or anything like that lately? Have you checked the TDS of your makeup water? It's possible your RO membrane has a hole in it.
With all the possible culprits it's amazing any of us can keep corals in the long term!
It could be a free metal or other toxin. Water changes and running some poly filter or cuprisorb might be useful.
My best guess at this point would be alkalinity and PO4 combined messing with calcification. But it's just a guess. The realtively high ph and relatively low alk might indicate an overdose of alkalinity spiking ph and resulting in precipitation which lowers the alk. Ime, many mystery stony coral issues have alk issues lurking in the background somewhere.
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