Because MCS is a sensitization that fluctuates, not a cured condition. People who have done well for years report being set back by a renovation, a new job, a flooded basement or a hidden exposure. Relapse is documented in the same communities that document recovery, and it is why "recovered" on this site always comes with "for some people" attached.
Relapse follows renewed exposure in most reported cases — often an exposure the person did not choose or did not know about until afterward. It also follows illness, surgery, major stress and, for some, nothing anyone can identify.
A relapse after a long remission tells you the threshold had risen and then fell again. It does not tell you the remission was false, and it does not tell you the person did something wrong.
Long-term cohorts describe MCS as chronic with fluctuation: a nine-year Iowa follow-up of eighteen people found them improved but most still symptomatic; the Japanese five-year population cohort found self-reported intolerance both improving and developing, with regular activity and lifestyle linked to improvement and negative psychosomatic states linked to onset. Nobody has measured what distinguishes people who relapse from people who hold their gains, or what preceded each relapse across a large group.
That is the recovery-phenotype question, and it is a dedicated study rather than a paragraph, because for patients it may matter more than proving what started the illness. The plausible variables — severity at onset, initiating exposure, comorbid MCAS or dysautonomia, ongoing hidden exposure, treatment of an underlying condition — are all untested against each other.
What preceded every relapse after improvement, in thousands of histories; whether relapsers and non-relapsers differ in initiator, comorbidity or course; and whether anything protects a remission.
Relapse is part of the natural history of MCS, it usually follows exposure, and it does not mean the improvement was imaginary or the person failed. Why some hold their gains and others do not is the question the Recovery Phenotype study exists to answer.