IME, zoanthids can absolutely melt away under a chemical warfare attack or starvation. However if you've been adding items and haven't been properly quarantining them, you could have introduced one of the common zoanthid eating pests like sundial snails or nudibranchs.
Regarding "mixed reef" tanks -- mixing a bunch of softies together is also a "mixed reef." Categories like softie, LPS and SPS are vague, invented labels that bear little relation to the care required for a particular coral. A deep water brown discosoma mushroom has entirely different light and flow needs than a Lobophyton leather coral that lives in shallow lagoons. The so-called SPS are more homogeneous in their care requirements than the other arbitrary groups, but they are still not all the same and they will absolutely war with each other.
If you want optimum growth, health and color, you keep a species specific tank. (Actually there are probably companion groupings that will produce better results, but I don't think anyone has done any real research on that.) Most of us don't want to keep just one species, and even a biotype tank is difficult to put together without breaking type. Nor is rampant growth always a good thing unless you are farming corals.
On the other end of the spectrum, it's possible to keep many different species in a tank, but you'll see sub-par health, growth and/or color and will likely lose some weak specimens from time to time. These tanks are more susceptible to crashes and coral disease outbreaks because everything is just getting by.
In general, if you don't want to keep one species or biotope, keep groups of animals which primarily thrive in the same kinds of conditions -- light, flow, water chemistry, turbidity, etc. Once you get things stabilized for the bulk of the creatures, there's always a nook or cranny that has different conditions in which you can place an odd coral.
If the goal is to keep SPS, you are probably better off removing most if not all of your softies -- particularly small specimens or colonies which will be more vulnerable -- or starting a second tank.
