You've been really thorough with your tank. Its hard to come up with an explanation but, there are tons of reasons something can go wrong. I've seen personally in the past acros just dying for no reason. More often than not, its been subtle mistakes like extra dosing too close to an autodose schedule, having something in the water building up over time that we can't test for (my main purpose for doing water changes), something nipping at corals (despite you not seeing it, not like you watch it 24 hours), something stinging/eating corals at night at it moves around, or other corals secreting battle chemicals near others. Not all acros can be near other acros. Be sure nothing has long tentacles nearby and there aren't any night time hitch-hikers.
Have you changed anything else that you aren't remebering? Added any new fish or inverts? Dosing any trace elements, phyto, or aminos?
Have you changed anything else that you aren't remebering? Added any new fish or inverts? Dosing any trace elements, phyto, or aminos?