This will be a bit rambling, but here goes.
I had almost the same thing happen after treating a tank with flucanzole. My thoughts were there was a vicious cycle going on, with several impacts combining to bring around catastrophic collapse. Impact one, is that despite the reported fluc MoA being disruption of ergosterol synthesis, it appears to impact some metabolic function of the coral or the algae partnership, as observed by the rapid decrease in uptake of calcium (I've actually used fluc twice, in two different tanks. One treatment turned out okay, the other a disaster; both times cal/alk uptake was negatively impacted). Whether this metabolic impact is direct or indirect, I have no idea, but there are hints in the literature (see paper below for one). Although ergosterol may not be a structural component of coral or zooxanthellae, it may be a vitamin precursor or maybe there may be toxin released from the death of the many protozoa and fungi that make up the biome of our tanks. Anyway, this swing in alk combined with whatever caused the metabolic swing in the first place is not good for coral health.
[nb, there are many fungi in our tanks - some seem beneficial for coral health - here is a free paper on the topic if you are interested (
https://www.frontiersin.org/articles/10.3389/fmicb.2014.00228/full). They talk about a wild colony of Acropora hyacinthus that appeared to have a beneficial relationship with a fungi and importantly that the fungi was metabolically important for the health of the acro, so I guess if teh same relationship were sometimes established in our aquaria, then fluc would be bad in some cases (there are three other papers referenced, but I don't have access to the journals).
Impact 2 - bacterial: I had the same second RTN event some time after arresting the first through large water changes. It may have been coincidence, but think the second one was because the fluc may have changed the microbiological population of the tank. It may have been coincidence, but I stopped the RTN by large water changes, installing UV, then turning off the UV and skimmer and adding a variety of bottled bacteria (Microbacter, two different Dr Tims - ecobalance and on-and-only) this seemed to work and the RTN stopped. (nb, the reason I suspect it was bacterial was that stupidly I took frags of some acros I didn't want to lose and put them in one of my other healthy systems. The same RTN then spread rapidly in the previously healthy tank. Could of course have been viral, but then I don't think the 'good' bacteria would have addressed the issue. The same fix worked for both tanks.
impact 3 dose - we often use fluc from pretty dodgy internet sources that probably don't have the best quality control in terms of uniform content, so who knows how much or even what you really dosed to the tank (50%, 100%, 400%, 4000%) or what the protocols posted online really dosed either (I appreciate this isn't an impact, but a confounding issue).