These blanket statements are not helpful. There are way too many variables in each tank to even make statements like this an absolute fact. I would argue that "maturity" of a tank is often not one of the main reasons for STN/RTN. 20 years of experience by the OP provides extra context in this case. Answers to posts like this must be specific.I think your tank isnt ready. I dont like this is only a 7 month old system. Even though you are seeding with live rock, how can everything including bacteria levels, ect. Be stable at 7mos to support acros.
It seems you are trying to fast forward the process of establishing a new tank. Start with digis, birdsnest add acropora later. Who knows maybe 1 dying acro in an unestablished system can cause the others to go down....i think you're paying the price for rushing delicate sps into the system for whatever reason.
I would keep adding cheap sps until this passes and or continue to check for metals and test parameters all the time since its a new system. Check for pests with lights off and uv light. Maybe nuke tank with dr. Gs but you are so unestablished not sure what the outcome will be....
I'm glad the OP seems to have found a cause. Good luck and sorry for you to have to deal with this.