For the reasons that you are experiencing, is why I am not planning to allow ANY soft corals in my new tank that is being built. Debating if I want to chance zoos on their own rock islands on the bottom.
Xenia are dependent on iodine. If your water column is lacking iodine, then xenia tend to disappear over time. The same for ULNS, running ULNS will cause softies to slowly disappear. Difficult to do if you have LPS though as you have to spot feed each LPS using this strategy.
Also I believe you are running skimmerless, which is also like steroids for softies. The higher DOC in the water will cause the softies to multiply at an alarming rate. Ask me how I know this
Dennis