I have the same problem. I really love torches and have 11 of them currently. 3 are doing really poorly with no flesh band and exposed septa or barely a flesh band just past the septa. I've probably bought another 5 that have since died. My tank is 13 months old and all my other corals including hammers, frogspawn, acans, fungias, favias, blastos, lots of acros, montis, mushrooms, zoas or others are thriving. Only my torches are doing poorly.
On mine it looks like the flesh bands are disintegrating and look powdery. I can see bone white skeleton all around the base of the polyp as well as the powdery white remains of the flesh. The tentacles will begin to shorten around the edge closest to the septa. This will continue until the flesh recedes all the way over the septa and then theres no flesh and only a polyp. They continue to go downhill from there until the head just dies or breaks out in brown jelly. This whole process takes months though, and I usually don't let it get that far and cut my losses and toss the coral.
I too really keep a good eye on my params with a Trident and I do Reef Moonshiners and do monthly ICPs. I've never had any major alk swings that I know of. My current params are below. I've tried the KFC dip and I even tried the in tank cipro treatment that Eli Meyer from Aquabiomics popularized. I think the cipro and KFC dips helped but it's hard to say for sure as it keeps happening.
The only things I'm adding to my tank are kalkwasser, Brightwell alk, Ca and Mg, Reef Moonshiner elements, live phyto (daily), microbacter7 (very rarely), Fauna Bacto Blend (once weekly), LRS reef frenzy, PE mysis, Reef Nutrition oyster feast and R.O.E., and TDO pellets.
I really want to get this figured out as I love torches and it's really the only issue I'm having with my tank keeping me between being happy and unhappy with my reef.
Current params:
Salinity: 34.8
pH: 8.37-8.5
Alk: 8.3-8.7
Ca: 435-440
Mg: 1430
PO4: 0.11
NO3: 9.7