I've been growing some Christmas cactus cuttings for about a year and they appear to spontaneously have patches where the cells just die. Until today, the plants were fine, but I walked past them five minutes ago, one of the stems had folded in the center. The whitish looking patch has been around for a few days, the brown-grey 'rot' is new.
I've read online that this can be a symptom of fungus or sunburn but to me, the spread is too irregular for fungus and the plant hasn't been receiving great sunlight. The first time I noticed the issue was when I was growing my plant at least 3m from the nearest window and with no direct sunlight. The problem only affects certain sections of the stem and appears very suddenly. The mother plant didn't have this problem either.