Sorry to hear that, what a bummer.
When starting up my 'new' system, I got an instant cycle because all of my old very established live rock and live stock was moved over. I assumed that all the maturity of my old system would carry over to this new system, despite one rock being new as well as new sand.
Maybe that isn't the case. I've ruled everything else out. I've done enough water changes to basically cycle out all old water over the course of a couple months.
So despite moving all my old live rock over, maybe a tank upgrade with new sand and one large new dead dry rock is enough to sort of reset the tank's life. In which case maybe this 'new' system is too young for acropora.
I'm getting algae blooms now, hair algae, cyano, and what looks like dinos as of two days ago -- algae blooms I'd associate with a new tank, blooms that were taken care of years ago in my old tank prior to moving everything over. It feels like a new tank starting from square 1.
Maybe this could account for acroporas dying. Even now, throwing cheap acros in, they perish. Softies are LPS are 100% healthy and happy. At this point I ruled everything else out. Now I wait for this new era of uglies to dissipate then I'll try again.
I did borrow a UV sterilizer from a buddy. I'm waiiting for a mj1200 to arrive in the mail today and I'll hook it up. I'm not anticipating anything amazing happening but at this point it couldn't hurt.