I've been gathering research on dendronephthya to support my own attempt at trying to keep these animals. I found an interesting study here:
https://www.sciencedirect.com/science/article/pii/S1978301916301589
It surveyed the bacterial colonies present on the coral once a week for four weeks in captivity and compared that to freshly collected specimens. The result I found compelling was that the numbers of actinobacteria plummeted in captivity to near 0 (not detected) from significant numbers. This type of bacteria is known to often create compounds which protect against pathogens.
Could our corals be losing their natural defense against disease and die as a result of that? This would certainly explain why even well fed specimens perish. The problem then is, what do we do about that? I'm still formulating a plan of attack.