I think the nutrients are your issue.
Yes, it is indeed possible to have thriving ULNS systems. But if you're not dosing all the amino acids constantly, I would suggest you start slowly raising your nitrate and phosphate and at least keep the tank "low nutrient" rather than pegging it at zero.
I went through this same exact thing with my tank, except the acros weren't dying, they just weren't growing and the colors weren't great. Ever since I raised my nutrients I've been seeing way better color and growth from all my corals.
I'm only at 2 ppm nitrates so you don't need to do a whole lot, you just don't want it pegged at zero. Follow the Redfield Ratio to keep your Nitrate and Phosphates in balance.
We tend to over-do our filtration and nutrient export. When I started my tank I was trying everything to keep my water clean. I had an overpowered skimmer, carbon and gfo, chaeto reactor, and microbubbling.
I've since learned that if your water is too clean, there is no food for the corals.
Fish poop is your best friend.
Now, I just run my skimmer and a tiny tiny bit of carbon and gfo.
EDIT:
Also, for alkalinity. Anything between 7-11 dkh is fine as long as you keep it stable.
But it's not always about your water parameters.. Make sure you have enough flow in your tank! Flow is a pretty big factor and SPS like a pretty good amount of flow. And yes lack of flow can cause SPS to die.