Sorry to her about your losses, but I can relate to the mystery SPS deaths. I ran into the same issues twice in two different tanks. Same thing as you, everything is fine, then slow losses, then a bit of recovery, slow losses again, etc. The only things I have done that seem to have ended those cycles was to install 2 new stages of chloramine removal in my RO/DI, and run a treatment of chemiclean. I looked into my City's water report and found they add chloramine at times to help purify the pipes. It would make sense to me that chemiclean is an antibiotic and can help if there is bacterial infection within the SPS. Just my .02$, hope you can recover.
Oh yes, this was my first hypothesis several years ago. Every year in October I would see a decline and I wondered if it correlated with when the city shocked the system. I added a dual stage RO system with dual DI, a sediment filter, and two additional chloramine blocks to make a total of a 9 stage RO/DI system.